First observation from the screenshots: The dojo.declare specifies another name than gets reported in the error message.
Did you specify the correct widget id in the
Unfortunately I cannot pinpoint the cause of the error in this specific situation.
I have had this error quite a few times myself. Even if the constructor is in the source, failure or exceptions during its execution also produce this error. Putting some console.log statements in the source may help you find out how far it actually gets.
Really frustrating at start but a life saver in the long run as the plugin detects common coding errors and forces a specific coding style that prevents a lot of misery.
I have had exactly this problem this morning, you should in fact just double check your .js for syntax error's. I found one and when fixed it solved my problem.
Make sure you write the correct code.