To reproduce:
Go to progress dialog
Add Billboard Host control
Place bitmap in it
Right click on the control and go to Edit Billboards
Create new, action: InstallFiles (I named it "BB2")
Click OK
Open the Properties drop-down combo
Click the bitmap you placed on first billboard
You get Runtime Error - R6025, pure virtual function call
I guess you're not re-initializing the drop-down combo when switching billboards (or creating new). The billboard that contains the bitmap is not currently opened and you may not have the instance of it available at this time.