Migrating from a 25 year old version of PowerBuilder is impressive. Most of the stuff should go through, but I am not surprised that there were at least a few errors.
To expound on what Mike said, if you look in your PBL, there will probably be a w_checker window, however, if you attempt to open w_checker in the window painter, PowerBuilder will pop up an error message likely saying that you need to regenerate it. When you attempt to regenerate it, you should get a better error indicating what the problem is. Possibly, there is something wrong with w_checker, possibly w_checker contains a reference to another object that did not migrate successfully.
So whenever you locate the lowest level window or object that failed to migrate properly, you can open it in source mode to fix the issue. Depending on the issue, you find it most convenient to comment out the offending code while editing in source mode which would allow you to save the window or object. At that point you can open it in the painter and fix it.
So once you've gotten the lowest level object fixed, you can try regenerating any dependent objects. It can be a bit tedious depending on the number of objects involved, but once you get into it, it should not be too bad.
