Bug 19075 Rename Java file to non Java file causes most actions to crash
F2 - open Java file in editor - rename it so that it has the extension .txt Almost all Java relevant actions in Edit, Source, Refactor, Naviagte and context menu are still enabled. Executing them almost always leads to some sort of exceptions.
org.eclipse.jdt.ui/ui/org/eclipse/jdt/internal/ui/javaeditor/CompilationUnitEditor.java
private void stopProblemIndication() {
		if (fProblemPainter != null) {
			fPaintManager.removePainter(fProblemPainter);
			fProblemPainter.deactivate(true);
			fProblemPainter.dispose();
			fProblemPainter= null;
		}
	}
	
	private boolean isProblemIndicationEnabled() {
		IPreferenceStore store= getPreferenceStore();
		return store.getBoolean(PROBLEM_INDICATION);
	}
	
	private void configureTabConverter() {
		if (fTabConverter != null) {
			IDocumentProvider provider= getDocumentProvider();
			if (provider instanceof CompilationUnitDocumentProvider) {
				CompilationUnitDocumentProvider cup= (CompilationUnitDocumentProvider) provider;
				fTabConverter.setLineTracker(cup.createLineTracker(getEditorInput()));
			}
		}
	}
	
	private int getTabSize() {
		Preferences preferences= JavaCore.getPlugin().getPluginPreferences();
		return preferences.getInt(CODE_FORMATTER_TAB_SIZE);	
	}
	
	private void startTabConversion() {