Crash when scroll on list with no items in it

[RubenKid]

When try to scroll on a list with no items, the library crashes

Stacktrace:

                                                                            Process: gvillani.it.pinnedlistdemo, PID: 30952
                                                                            java.lang.ArrayIndexOutOfBoundsException: length=0; index=-1
                                                                                at java.util.ArrayList.get(ArrayList.java:310)
                                                                                at com.gvillani.pinnedlist.GroupListWrapper.getItemPinned(GroupListWrapper.java:56)
                                                                                at com.gvillani.pinnedlist.PinnedAdapter.getItem(PinnedAdapter.java:66)
                                                                                at com.gvillani.pinnedlist.PinBehavior.updatePinContent(PinBehavior.java:288)
                                                                                at com.gvillani.pinnedlist.PinBehavior.initPin(PinBehavior.java:315)
                                                                                at com.gvillani.pinnedlist.PinBehavior.onNestedPreScroll(PinBehavior.java:186)
                                                                                at android.support.design.widget.CoordinatorLayout.onNestedPreScroll(CoordinatorLayout.java:1743)
                                                                                at android.support.v4.view.ViewParentCompatLollipop.onNestedPreScroll(ViewParentCompatLollipop.java:70)
                                                                                at android.support.v4.view.ViewParentCompat$ViewParentCompatLollipopImpl.onNestedPreScroll(ViewParentCompat.java:184)
                                                                                at android.support.v4.view.ViewParentCompat.onNestedPreScroll(ViewParentCompat.java:353)
                                                                                at android.support.v4.view.NestedScrollingChildHelper.dispatchNestedPreScroll(NestedScrollingChildHelper.java:209)
                                                                                at android.support.v7.widget.RecyclerView.dispatchNestedPreScroll(RecyclerView.java:10162)
                                                                                at android.support.v7.widget.RecyclerView.onTouchEvent(RecyclerView.java:2723)
                                                                                at android.view.View.dispatchTouchEvent(View.java:9993)
                                                                                at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2828)
                                                                                at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2499)
                                                                                at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2839)
                                                                                at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2514)
                                                                                at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2839)
                                                                                at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2514)
                                                                                at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2839)
                                                                                at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2514)
                                                                                at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2839)
                                                                                at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2514)
                                                                                at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2839)
                                                                                at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2514)
                                                                                at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2839)
                                                                                at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2514)
                                                                                at com.android.internal.policy.PhoneWindow$DecorView.superDispatchTouchEvent(PhoneWindow.java:2831)
                                                                                at com.android.internal.policy.PhoneWindow.superDispatchTouchEvent(PhoneWindow.java:1863)
                                                                                at android.app.Activity.dispatchTouchEvent(Activity.java:3046)
                                                                                at android.support.v7.view.WindowCallbackWrapper.dispatchTouchEvent(WindowCallbackWrapper.java:67)
                                                                                at android.support.v7.view.WindowCallbackWrapper.dispatchTouchEvent(WindowCallbackWrapper.java:67)
                                                                                at com.android.internal.policy.PhoneWindow$DecorView.dispatchTouchEvent(PhoneWindow.java:2792)
                                                                                at android.view.View.dispatchPointerEvent(View.java:10228)
                                                                                at android.view.ViewRootImpl$ViewPostImeInputStage.processPointerEvent(ViewRootImpl.java:5350)
                                                                                at android.view.ViewRootImpl$ViewPostImeInputStage.onProcess(ViewRootImpl.java:5186)
                                                                                at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:4626)
                                                                                at android.view.ViewRootImpl$InputStage.onDeliverToNext(ViewRootImpl.java:4679)
                                                                                at android.view.ViewRootImpl$InputStage.forward(ViewRootImpl.java:4645)
                                                                                at android.view.ViewRootImpl$AsyncInputStage.forward(ViewRootImpl.java:4787)
                                                                                at android.view.ViewRootImpl$InputStage.apply(ViewRootImpl.java:4653)
                                                                                at android.view.ViewRootImpl$AsyncInputStage.apply(ViewRootImpl.java:4844)
                                                                                at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:4626)
                                                                                at android.view.ViewRootImpl$InputStage.onDeliverToNext(ViewRootImpl.java:4679)
                                                                                at android.view.ViewRootImpl$InputStage.forward(ViewRootImpl.java:4645)
                                                                                at android.view.ViewRootImpl$InputStage.apply(ViewRootImpl.java:4653)
                                                                                at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:4626)
                                                                                at android.view.ViewRootImpl.deliverInputEvent(ViewRootImpl.java:7312)
                                                                            	at android.view.ViewRootImpl.d