fpspreadsheet: Fix manualtest crashing if started a second time from the same gui session.

git-svn-id: https://svn.code.sf.net/p/lazarus-ccr/svn@3280 8e941d3f-bd1b-0410-a28a-d453659cc2b4

components/fpspreadsheet/tests/manualtests.pas

@@ -153,6 +153,7 @@ begin
   if Workbook <> nil then begin
     Workbook.WriteToFile(TestFileManual, OUTPUT_FORMAT, TRUE);
     Workbook.Free;
+    Workbook := nil;
   end;
 end;