Exact steps for others to reproduce the error
If I have a skeletal rig posed with keyframes, and I want to select a channel of a Shader Nodetree, it's impossible.
For some reason clicking on the channel or the keyframes fails to activate the fcurve, so it's impossible to add modifiers in the graph editor.
It sounds bizarre, but I've spent over 2 hours refining this bug to its core, and that appears to be its simplest description.
Please see attached blend example below.
To test, try selecting any of the shader tree property channels, observe that the channels cannot be selected. Now delete rig and try again. Notice that the channels are now selectable.
Pushing the rig action into a strip or hiding it also allows the shader channels to be selected.