LXReorderableCollectionViewFlowLayout
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Fixed intermittent crash when long press one cell & drag and tap/hold an...
...other cell. When local variable 'previousIndexPath' is nil, the method -(void) invalidateLayoutIfNecessary will crash at line 171. This fix prevents indexing into 'nil' at line 171.
Reference for future review: https://github.com/lukescott/DraggableCollectionView/issues/37