System Information
Operating system: Linux-4.15.0-142-generic-x86_64-with-glibc2.23 64 Bits
Graphics card: NVIDIA GeForce GTX 1070 Ti/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 465.24.02

Blender Version
Broken: version: 2.93.2 Release Candidate, branch: master, commit date: 2021-07-27 12:05, hash: rB45ed762124c5
Worked: None

Short description of error
An Existing Geometry Nodes node tree disappears from the GN Editor when other modifiers are added to the same object

Exact steps for others to reproduce the error

1 - Add a Geometry Nodes modifier to the default cube.

2 - Insert a Point Distribute node to the node tree.

3 - Add an array modifier (or any other modifier) to the same object and the node tree in the Geometry Nodes editor disappears, but still works.

If the other modifier is removed, the node tree re-appears. The problem does not occur if you add the other modifiers first then add the geometry node modifier. Re-ordering the modifiers in the modifier stack or disabling them makes no difference.

You can add a new GN modifier and select the pre-existing node tree then delete the first GN modifier as a workaround.

This also happens in Blender version 3.0.