[UI] Viewport display only perspective view instead of top, front and right after switching layout to 4 Split.

Viewport display only perspective view instead of top, front and right after switching layout to 4 Split.

Steps to repro:
1. Open the attached Project.
2. Change the Layout to 4 Split.

Actual Result:
Viewport display only perspective instead of top, front and right view.

Expected Result:
Viewport display top, front and right view.

Reproducible in:
2019.1.0a13, 2019.1.0a11, 2019.1.0a9.

Working Fine in:
2019.1.a8, 2018.3.0f2 (49866f0a5d12).

Environment:
Windows & Mac.