It's just a deprecation warning for some widget.
You can try to update your widgets or manually check which one is still using this.
But generally I wouldn't worry too much about that until Mendix 9 arrives.
It means an exception is thrown because of a variable being null. Mendix tried to handle it with a function that is deprecated and you are advised to use it's improved version, being plain javascript.
So indeed nothing to worry about. And if you want to solve it now, check your widgets, especially your javascript widgets that make use of the function mendix.lang