When the UI Toolkit Sample window is docked in the editor and scene view or game view is maximized and minimized then the docked UI Toolkit Sample window disappears with the following Errors, Also when the UI Toolkit Sample window is maximized and minimized, only the 1st error mentioned below appears in the console.
"Invalid editor window UnityEditor.FallbackEditorWindow
UnityEditor.GenericMenu:CatchMenu (object,string[],int)"

"Invalid editor window UnityEditor.FallbackEditorWindow
UnityEditor.EditorApplication:Internal_CallGlobalEventHandler ()".

Steps to repro:
1. Create a new project
2. Window > UI Toolkit > Sample
3. Docked UI Toolkit window
4. Maximize UI Toolkit window
5. Maximize Scene View or Game View

Actual Result:
On maximizing UI Toolkit Samples window error is thrown and maximizing Scene View OR Game View, UI Toolkit Samples window disappears along with the error

Expected Result:
UI Toolkit Samples window maximize gracefully and window remains docked on maximizing and minimizing Scene View OR Game View

Note: UIElements Sample window is introduced in 2019.2

Occurring on:
2020.2.0a11, 2020.1.0b8, 2019.3.12f1, 2019.2.13f1

Environment:
Windows 10 and macOS 10.14