This error usually means than the designer can't find information for some widget inside parent component (content pane or panel). And usually this doesn't mean that WBP could not locate the class definition for a custom component but may be there are more exceptions in log?
Actually, component id = 95 means that your gui is very complex, since ids starts from zero
but GL support should work anyway so the test case would be very appreciated. And for now until we fix this problem you may try to use another Swing layout.
The "best practices" of migrating to WBP depends on initial code. If the code is too complex and messy then sometimes its better to create new gui on scratch, this also may give the better result because of eliminate the disadvantages of old gui.