Operating system: Windows
(example: 2.80, rB1df6a98b176f, blender2.8, 2019-03-05)
Short description of error
As soon as a Material uses node groups with ImageTexture Nodes, those image textures cannot be selected from the TextureSlot Dropdown.
Also please refer to my comment here:
Exact steps for others to reproduce the error
Create Suzanne. Go into Texture Paint Tab. Create two texture slots. -> selection works.
Open Shader Editor. Group those 2 textures into a new group. -> selection doesn't work.