Brightness/Contrast histogram doesn't always update when switching between similar images

[petebankhead]

Bug report

Describe the bug
When switching between images with similar channels with the Brightness/Contrast window open, the histogram doesn't automatically update as it should. Rather, the user sometimes need to select a different channel and then return to the one they wanted to look at.

To Reproduce
Steps to reproduce the behavior:

1. Create a project with some similar images (same channels, names)
2. Open Brightness/Contrast
3. Open the first image, see the histogram displayed
4. Open the second image; the histogram from the first probably still lingers

Expected behavior
The histogram updates when the image updates.

Desktop (please complete the following information):

• OS: macOS (but presumably all)
• QuPath Version: 0.5.0

Additional context
The 'Apply to similar images' setting doesn't seem to make any difference to this behavior.

The issue isn't evident if the images have different channels.