set position block field editor moves when modified

[jwunderl]

Describe the bug

The field editor div on the set position block 'jumps' occasionally when the slider changes

@dmonroym noticed that this seems to only happen when mySprite is not defined, so it may be the case that it is just inconsistently including the warning symbol when rerendering

To Reproduce Steps to reproduce the behavior:

1. Go to arcade.makecode.com project
2. drag out set mysprite position to (as only block in on start)
3. play with field editor
4. See error

Expected behavior

should stay still