When references computation is started in parallel, the later one does not have progressbar. The same problem happens for computation of GR roots and retained size.

Steps to reproduce the behavior:

1. Open two heap dumps.
2. Start computation of references in the first heap dump.
3. While computation is still running, switch to second heap dump.
4. Start computation of references in the second heap dump.
5. Progressbar for first computation finishes, but the second one is never displayed.
6. References for the second heap dump are computed without progressbar.

Expected behavior
There should be progressbar when references are computed.