Backdrop number stage monitor don't appear after creating new project

[apple502j]

Expected Behavior

Backdrop number stage monitor appears when checkbox is on

Actual Behavior

Backdrop number stage monitor does not appear the first time, but if you un-check and check again, it appears

This may have something to do with #2324

Steps to Reproduce

• Show "backdrop number" stage monitor
• click New
• Click the checkbox for showing backdrop number stage monitor again
• It doesn't appear and console has one error
• Un-check and check again
• It appears

TypeError: stage is undefined scratch3_looks.js:569
    getBackdropNumberName scratch3_looks.js:569
    getBackdropNumberName self-hosted:878
    execute execute.js:523
    stepThread sequencer.js:212
    stepThreads sequencer.js:128
    _step runtime.js:2031
    _steppingInterval runtime.js:2552

Operating System and Browser

Windows 10, Firefox 74