Making costume empty in paint editor doesn't update stage right away

[towerofnix]

Expected Behavior

Any changes in the paint editor should be immediately reflected on the stage.

Actual Behavior

If you make a costume empty through the paint editor (i.e. by deleting or erasing the last remaining shapes), the stage won't immediately be updated. (Once some other change is made, and the stage does update, it'll reflect the shapes having been deleted.)

Steps to Reproduce

Delete the last shape in a costume editor, so that the costume is empty. Note that the stage doesn't update until you make some other change.

Operating System and Browser

Firefox 66.0b8.

[ntlrsk]

The deleted costume staying on the stage is happening even after adding another sprite and deleting that costume, both images still appear on the stage, even though the sprites/costumes are empty.

To reproduce:

1. Delete the cat costumes.
2. Choose a new sprite.
3. Delete its costumes.
   The images will still appear on the stage:
   

[ntlrsk]

I'm also having the opposite problem, I pasted an image and it's not updating on the stage or the costume editor, but you can see it in the thumbnail. (this remains true even after I go back to the code tab):

[fsih]

Moved to scratchfoundation/scratch-paint#901