System Information
Operating system: Linux-5.4.0-54-generic-x86_64-with-debian-buster-sid 64 Bits
Graphics card: GeForce GT 630/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 390.138

Blender Version
Broken: version: 2.92.0 Alpha, branch: geometry-nodes, commit date: 2020-11-24 13:30, hash: rB49f1c8ebecae
Worked: (newest version of Blender that worked as expected)

Short description of error
Switching from Shader- to Geometry node editor shows still shader

Exact steps for others to reproduce the error

Create a shader nodetree
Switch the shader editor window into a Geometry node editor
The text in the left bottom corner flickers and there is no geometry node setup, but still the shader

You can x out the shader name in the header, and the geometry start- and endnodes appear, but only in the editor, not in the modifier list: