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
return;
		
		if (DEBUG)
			System.out.println(DEBUG_PREFIX + "disposing AST: " + toString(fAST)); 
		
		fAST.setProperty(AST_DISPOSED, Boolean.TRUE);
		fAST= null;
		
		cache(null, null);
	}
	/**
	 * Returns a string for the given Java element used for debugging.
	 * 
	 * @param javaElement the compilation unit AST
	 * @return a string used for debugging
	 */
	private String toString(IJavaElement javaElement) {
		if (javaElement == null)
			return "null"; 
		else
			return javaElement.getElementName();
		
	}
	
	/**
	 * Returns a string for the given AST used for debugging.
	 * 
	 * @param ast the compilation unit AST
	 * @return a string used for debugging
	 */