I just spent an hour trying to fix this error. When I finally did, I realized it was a super easy problem and should have only taken 10 seconds to fix. Hopefully this post helps some else out there…
I’m editing a Flash file that was created by someone else (which is why I was blindsided by this issue). All the ActionScript is in a number of class files….nothing fancy, though. I added an image to the stage and converted it to a MovieClip. Then I gave it an instance name since I was going to write a little code to control it. As soon as I gave it an instance name, I got this nasty runtime error:
Keep in mind, I haven’t added/changed any ActionScript yet. Giving it an instance name caused the error. Without an instance name, everything worked fine. I was in my morning haze, so a couple cups of coffee later, I remembered why this happens.
You will get this error if Automatically declare stage instances is unchecked in your publish settings. You can find this checkbox by going to File > Publish Settings > Flash and clicking the Settings… button next to ActionScript 3.0:
Checking this box will fix the issue. If you don’t have this box check, that mean you have to declare any instance names in your code:
Hope this helps!