[HeapViewer] Cannot call progress on a task that was never started

Repeated warning in the console log:
```
WARNING [org.netbeans.modules.progress.spi.InternalHandle]: Cannot call progress on a task that was never started at org.graalvm.visualvm.heapviewer.HeapFragment$1.actionPerformed(HeapFragment.java:77)
```


Steps to reproduce:
1. Reopen heap dump, where path to GC root was computed.
