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
* Sets the given message as error message to this editor's status line.
	 * 
	 * @param msg message to be set
	 */
	protected void setStatusLineErrorMessage(String msg) {
		IEditorStatusLine statusLine= (IEditorStatusLine) getAdapter(IEditorStatusLine.class);
		if (statusLine != null)
			statusLine.setMessage(true, msg, null);	
	}
	/**
	 * Sets the given message as message to this editor's status line.
	 * 
	 * @param msg message to be set
	 * @since 3.0
	 */
	protected void setStatusLineMessage(String msg) {
		IEditorStatusLine statusLine= (IEditorStatusLine) getAdapter(IEditorStatusLine.class);
		if (statusLine != null)
			statusLine.setMessage(false, msg, null);	
	}
	
	/**
	 * Returns the signed current selection.
	 * The length will be negative if the resulting selection
	 * is right-to-left (RtoL).
	 * <p>
	 * The selection offset is model based.
	 * </p>
	 * 
	 * @param sourceViewer the source viewer