Reproduction steps:
1. Create and open a new project with the Universal Render Pipeline template
2. In the project window, go to "ExampleAssets" > "Shaders" and open the "Paint 1G_W Anim_Shader" shader graph
3. Undock the ShaderGraph window
4. Create any new node
5. Close the ShaderGraph window
6. In the dialog notifying that ShaderGraph has been modified, press "Cancel"
7. Repeat step 6

Expected result: The shader graph window does not get docked into the Editor window
Actual result: The shader graph window gets docked into the Editor window

Reproducible with: 2020.2.0b10 (10.1.0), 2020.2.7f1 (10.3.1), 2020.3.0f1 (10.3.1), 2021.1.0b11 (11.0.0), 2021.2.0a8 (12.0.0)
Not reproducible with: 2018.4, 2019.4, 2020.2.0b9 (10.0.0-preview.27)