The expression "app.x" is not a variable name. In this context it is an expression referring to the property x of an object stored in the variable app. [There are other potential meanings, but that's the one that's relevant here.]
If you want to check if the object stored in the variable app has a property x, use isprop.
isprop(app, 'x')
If you want to check if the property app.x exists and is non-empty:
isprop(app, 'x') && ~isempty(app.x)
If you know that the object app must have a property x (because it appears in the class definition file rather than being a dynamic property added using addprop on a class that subclasses dynamicprops) you can omit the first part of that test.
~isempty(app.x)
