The short answer is yes, you need to do this for each such bug you find, and if you have a commercial license, you need to do it yourself - the only way to distribute fixes for things like this is under GPLv2, which may not be something you want in your project as a dependency.
If this is a gwt compiler error, editing bytecode is not sufficient, as GWT consumes the source each time it builds. You probably should also update the bytecode if you update the source files. Only the file in question needs to be edited in this way (HtmlEditor, ListField are the ones I know of).
Do not replace with a .toString() call, in case the model holds an explicit null value, casting to object will use StringBuilder.append(Object), which will be null safe, or pick some other null safe approach.
Sencha's release policy at the time was that they supported the last major release - this means that until GXT 4 was released, GXT 2 was supported, and got updates, but once 4.0.0 was available, GXT 2 was no longer supported. To my knowledge, GXT isn't receiving any updates at all any more. Combined with the license issues, it is hard to do more than discuss the possible fixes and how they could be applied.