Hello! I've had this problem too. Do you find that whatever you click on, the menu bar disappears from the right or left? This can happen if you've accidentally set the Hotkey for one of the costumes to your left click button. This can happen accidentally pretty easily.
1. Exit out of Edit mode
2. Click the cog wheel on the right
3. Scroll through the Costume Keys and make sure they're either set to a hotkey, or set to Null. Makesure it's not set to something that says something like "Keybind1." If it is, delete that hotkey.