Bug 75711 AstProvider should not create AST if requested by SelectionListenerWithASTManager
3.1 M2 If SelectionListenerWithASTManager waits for an AST and the editor closes we should not create an AST for that one but return null instead.
org.eclipse.jdt.ui/ui/org/eclipse/jdt/internal/ui/javaeditor/JavaEditor.java
/**
		 * Creates a new delete previous sub-word action.
		 */
		public DeletePreviousSubWordAction() {
			super(ST.DELETE_WORD_PREVIOUS);
		}
		/*
		 * @see org.eclipse.jdt.internal.ui.javaeditor.JavaEditor.PreviousSubWordAction#setCaretPosition(int)
		 */
		protected void setCaretPosition(int position) {
			if (!validateEditorInputState())
				return;
			
			final int length;
			final ISourceViewer viewer= getSourceViewer();
			Point selection= viewer.getSelectedRange();
			if (selection.y != 0) {
				position= selection.x;
				length= selection.y;