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/ASTProvider.java
fWorkbench.addWindowListener(fActivationListener);
		
		
		IWorkbenchWindow[] windows= fWorkbench.getWorkbenchWindows();
		for (int i= 0, length= windows.length; i < length; i++)
			windows[i].getPartService().addPartListener(fActivationListener);
	}
	
	private void activeJavaEditorChanged(IWorkbenchPart editor) {
		fActiveEditor= editor;
		IJavaElement javaElement= null;
		if (editor != null)
			javaElement= ((JavaEditor)editor).getInputJavaElement();
		
		if (DEBUG)
			System.out.println(DEBUG_PREFIX + "active editor is: " + toString(javaElement)); 
		synchronized (fReconcileLock) {
			if (fIsReconciling && !fReconcilingJavaElement.equals(javaElement)) {
				fIsReconciling= false;
				fReconcilingJavaElement= null;
			}
		}
		cache(null, javaElement);
	}
	/**
	 * Returns whether the given compilation unit AST is
	 * cached by this AST provided.
	 * 
	 * @param ast the compilation unit AST