Thread-selection drop-down does not open in Hierarchy mode, limiting it to showind the Main Thread only.
ArgumentOutOfRangeException: Index was out of range. Must be non-negative and less than the size of the collection exception is thrown on selecting Main thread window.

Steps to repro:
1. Create a new project.
2. Windows > Analysis > Profiler.
3. Enter Play Mode.
4. Select Hierarchy View, click on the Thread-selection drop-down (labeled "Main thread" by default)

Actual Result:
Drop-down does not open.
ArgumentOutOfRangeException is thrown in the console.

Expected Result:
Thread-selection drop-down opens up; another thread can be selected.

Reproducible in:
2019.3.0a12.

Working Fine in:
2019.3.0a11.

Environment:
Windows & Mac 10.14.

NOTE: Please find attached stack trace file.