1. Create a new project.
2. Install ProBuilder through the package manager.
3. Open the ProBuilder window. (Tools -> ProBuilder -> ProBuilder Window)
4. Open the UV Editor window. (ProBuilder Window -> UV Editor)
5. Dock the UV Editor window anywhere.

Expected Outcome: No issues, no errors.
Actual Outcome: Continuous errors in the console.

Reproducible in 2020.1.0a22 with ProBuilder preview.2 4.3.0, 4.1.0. Doesn't seem to be package specific.
Not reproducible in 2020.1.0a21 with ProBuilder preview.2 4.3.0

Error Message:
Assertion failed on expression: '!dest.m_MultiFrameGUIState.m_Windows'

Notes:
- Tested on Windows and Mac.
- Errors can be cleared, but will continue to print to console until you restart the project.
- GIF attached demonstrating the issue.
- Tried other Probuilder windows, no apparent issues.
- Tried other dockable editor windows, no apparent issues. Unsure if there are any other windows affected by this.