Resynthisizer Heal Selection Runtime Error

[gstalnaker]

I'm using GIMP 2.10.8 (updated because of this issue) on Windows 10 Home Edition, with the 64-bit version of the Resynthesizer plugins/execs. I've used this plugin very frequently in the past with GIMP 2.8.x for fixing damaged BW images (family photos from the 50s/60s). My normal process for these RGB, 16-bit grayscale images is to:

1. use Wavelet Decompose to create detail layers
   
2. dup a detail layer
   
3. use Levels to enhance the damage on the dupped layer
   
4. paint over the damage on the dupped layer with color X
   
5. select by color X
   
6. click on the original detail layer so selection applies to it
   
7. run Heal Selection.
   

Now when I do this the Heal Selection process throws an error (see attached image): "RuntimeError: The texture source is empty. Does any selection include non-transparent pixels?"
I dimly recall having an issue like this in the past. I definitely have the right layer selected and so far as I know (!) there are only non-transparent pixels in the select. The layer after decomposition is done is in mode Grain Merge. That was in the past no issue.

Anyone with any ideas? I'm stumped.