Edit in Place grayed out for static text symbol

I'm using Animate CC 2018 on Windows 7 Pro 64bit.  I ran into a problem with not being able to use the Edit in Place command (was grayed out). The symbol I wanted to edit was a simple movieclip containing only one block of static 2D text, one frame.  In searching for the answer, I found an old post from 2009 saying that the problem was due to the symbol containing 3D animation, but this was not the case.  No animation, no 3D, no nested MC, nothing except a block of text.  In this case, it was no big deal to break it apart and re-save the MC, and a more complex MC could be edited by double-clicking on the Library item, but I'm hoping to find the reason and solution, as this command is very useful when it works.  Thank you for your help!