Steps to reproduce:

1. Open VisualEditor
2. Go to Insert>Media
3. Switch to "Upload" tab
4. Select an image to upload
5. Check "This is my own work" and click on "Upload"

6.Fill up the required fields and click on "Save"
7.Click on "Use this image"
8.Go to Advanced Settings
9.Click on Custom tab for Image size

Observed Result:
It crashes and there is an error in the console:
TypeError: dimensions is null