FIX: Possible crash on form close

src/ffileproperties.pas

@@ -868,6 +868,7 @@ begin
   if Assigned(FFileSourceCalcStatisticsOperation) then
   begin
     tmUpdateFolderSize.Enabled:= False;
+    FFileSourceCalcStatisticsOperation.RemoveStateChangedListener([fsosStopped], @FileSourceOperationStateChangedNotify);
     FFileSourceCalcStatisticsOperation.Stop;
   end;
   FFileSourceCalcStatisticsOperation:= nil;

src/fviewer.pas

@@ -2343,6 +2343,7 @@ begin
   if Assigned(FFileSourceCalcStatisticsOperation) then
   begin
     tmUpdateFolderSize.Enabled:= False;
+    FFileSourceCalcStatisticsOperation.RemoveStateChangedListener([fsosStopped], @FileSourceOperationStateChangedNotify);
     FFileSourceCalcStatisticsOperation.Stop;
   end;
   FFileSourceCalcStatisticsOperation:= nil;