RosterWindow: Fix Index out of range

Currently, detecting a file drop on the roster window is not recognized
correctly. This fix avoids an Index error thrown after dropping a file
on the roster by checking if there is a selection before continuing.
3 jobs for roster-drop-fix in 2 minutes and 27 seconds (queued for 2 minutes and 31 seconds)
Status Job ID Name Coverage
  Test
passed #11336
run-appdata

00:00:05

passed #11335
run-tests

00:02:12

 
  Build
passed #11337
run-build

00:00:09