Removing custom url of particles breaks it

[Benjythebee]

Describe the bug If you remove the url of a custom image on a particle feature, the feature breaks and won't work anymore. It becomes Undefined

To Reproduce Steps to reproduce the behavior:

1. Create a particle object
2. Assign url of custom image
3. refresh and then delete image url from UI
4. item won't work, even after more refreshes

Expected behavior The particle feature should revert back to normal and work.

Screenshots

Desktop (please complete the following information):

• OS: windows 10
• Browser Chrome
• Version 83