Missing backgrounds cause editor crash on asset reload.

[RichardULZ]

Describe the bug
Missing backgrounds cause editor to crash on asset reload, even if the assets have been fixed.

To Reproduce
Steps to reproduce the behavior:

1. Delete or rename a background
2. Asset is auto reloaded with blank white 256x256 square, (no path name).
3. Undo the delete / rename, outside of editor, or leave it missing (fix both cases)
4. Try to manually reload assets.
5. Editor crash with "path" argument must be of type string. Received type undefined path.js:L39C:5

Expected behavior
Background is found and reloaded nicely, or left missing, no crash.
Dimensions and collisions are retained while the file is missing (test if they persist through save),
Actor positions are retained (currently fine, may break with #386).

Not important, perhaps text saying Background "path/name" missing inside the blank background?

Screenshots
If applicable, add screenshots to help explain your problem.

Platform (please complete the following information):

• OS: [Windows 10]
• App version [1.2.1 / dev]

Additional context
Similar to #386

[RichardULZ]

This no longer crashes the editor, but will not find the background again once it is replaced, tough to fix.