Obscure way to get an error
Select an object then double-click Format Painter to style multiple objects.
Don't hit ESC to kill Format Painter
Close the .story but don't exit Storyline
Re-load the .story or load another project. Look at the cursor. Format Painter is still on.
Click any object. Bam! Error.
Not a big deal once you realize Format Painter is on. Hit ESC and you're good.
The bug here is loading a .story should reset Format Painter to off.