Android: NPE in Activity Preview Window.

[FroMage]

With a Ceylon project with a single Ceylon activity, if I open the activity_main.xml file I get this exception in the preview window:

java.lang.NullPointerException
    at android.support.v7.app.WindowDecorActionBar.getDecorToolbar(WindowDecorActionBar.java:241)
    at android.support.v7.app.WindowDecorActionBar.init(WindowDecorActionBar.java:197)
    at android.support.v7.app.WindowDecorActionBar.<init>(WindowDecorActionBar.java:189)
    at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
    at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)
    at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
    at java.lang.reflect.Constructor.newInstance(Constructor.java:422)
    at org.jetbrains.android.uipreview.ViewLoader.createNewInstance(ViewLoader.java:465)
    at org.jetbrains.android.uipreview.ViewLoader.loadClass(ViewLoader.java:172)
    at org.jetbrains.android.uipreview.ViewLoader.loadView(ViewLoader.java:105)
    at com.android.tools.idea.rendering.LayoutlibCallbackImpl.loadView(LayoutlibCallbackImpl.java:176)
    at com.android.layoutlib.bridge.bars.AppCompatActionBar.<init>(AppCompatActionBar.java:89)
    at com.android.layoutlib.bridge.impl.Layout.createActionBar(Layout.java:237)
    at com.android.layoutlib.bridge.impl.Layout.<init>(Layout.java:151)
    at com.android.layoutlib.bridge.impl.RenderSessionImpl.inflate(RenderSessionImpl.java:213)
    at com.android.layoutlib.bridge.Bridge.createSession(Bridge.java:426)
    at com.android.ide.common.rendering.LayoutLibrary.createSession(LayoutLibrary.java:350)
    at com.android.tools.idea.rendering.RenderTask$2.compute(RenderTask.java:520)
    at com.android.tools.idea.rendering.RenderTask$2.compute(RenderTask.java:508)
    at com.intellij.openapi.application.impl.ApplicationImpl.runReadAction(ApplicationImpl.java:967)
    at com.android.tools.idea.rendering.RenderTask.createRenderSession(RenderTask.java:508)
    at com.android.tools.idea.rendering.RenderTask.access$600(RenderTask.java:75)
    at com.android.tools.idea.rendering.RenderTask$3.call(RenderTask.java:620)
    at com.android.tools.idea.rendering.RenderTask$3.call(RenderTask.java:617)
    at com.android.tools.idea.rendering.RenderService.runRenderAction(RenderService.java:371)
    at com.android.tools.idea.rendering.RenderTask.render(RenderTask.java:617)
    at com.android.tools.idea.rendering.RenderTask.render(RenderTask.java:639)
    at org.jetbrains.android.uipreview.AndroidLayoutPreviewToolWindowManager.doRender(AndroidLayoutPreviewToolWindowManager.java:654)
    at org.jetbrains.android.uipreview.AndroidLayoutPreviewToolWindowManager.access$1700(AndroidLayoutPreviewToolWindowManager.java:82)
    at org.jetbrains.android.uipreview.AndroidLayoutPreviewToolWindowManager$7$1.run(AndroidLayoutPreviewToolWindowManager.java:596)
    at com.intellij.openapi.progress.impl.CoreProgressManager$2.run(CoreProgressManager.java:142)
    at com.intellij.openapi.progress.impl.CoreProgressManager.registerIndicatorAndRun(CoreProgressManager.java:446)
    at com.intellij.openapi.progress.impl.CoreProgressManager.executeProcessUnderProgress(CoreProgressManager.java:392)
    at com.intellij.openapi.progress.impl.ProgressManagerImpl.executeProcessUnderProgress(ProgressManagerImpl.java:54)
    at com.intellij.openapi.progress.impl.CoreProgressManager.runProcess(CoreProgressManager.java:127)
    at org.jetbrains.android.uipreview.AndroidLayoutPreviewToolWindowManager$7.run(AndroidLayoutPreviewToolWindowManager.java:591)
    at com.intellij.util.ui.update.MergingUpdateQueue.execute(MergingUpdateQueue.java:337)
    at com.intellij.util.ui.update.MergingUpdateQueue.execute(MergingUpdateQueue.java:327)
    at com.intellij.util.ui.update.MergingUpdateQueue$3.run(MergingUpdateQueue.java:271)
    at com.intellij.util.ui.update.MergingUpdateQueue.flush(MergingUpdateQueue.java:286)
    at com.intellij.util.ui.update.MergingUpdateQueue.flush(MergingUpdateQueue.java:244)
    at com.intellij.util.ui.update.MergingUpdateQueue.run(MergingUpdateQueue.java:234)
    at com.intellij.util.concurrency.QueueProcessor.runSafely(QueueProcessor.java:238)
    at com.intellij.util.Alarm$Request$1.run(Alarm.java:352)
    at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
    at java.util.concurrent.FutureTask.run(FutureTask.java:266)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
    at java.lang.Thread.run(Thread.java:745)

Apparently that's because it tries to invoke the Ceylon activity to see how it renders. Perhaps it's related to the fact that it can't "see" the activity class in question. Or it needs something else set up? I've no idea how we can trick AS into running it, frankly, but this looks very much like the plugin has crashed. We should at least investigate what is the cause of this and if it's fixable and how much effort it would take.