CppunitTest_sw_tiledrendering: close the document while LOK is still active

This is the same type of problem that was fixed for
CppunitTest_desktop_lib in commit
da17ccae (CppunitTest_desktop_lib: close
the document while LOK is still active, 2019-05-09).

Change-Id: I2fe050bf40d68dafa0334f4149e54958c492122d
Reviewed-on: https://gerrit.libreoffice.org/72126Reviewed-by: Miklos Vajna <vmiklos@collabora.com>
Tested-by: Jenkins

sfx2/source/view/viewsh.cxx

@@ -1458,6 +1458,9 @@ void SfxViewShell::registerLibreOfficeKitViewCallback(LibreOfficeKitCallback pCa
 
     afterCallbackRegistered();
 
+    if (!pCallback)
+        return;
+
     // Ask other views to tell us about their cursors.
     SfxViewShell* pViewShell = SfxViewShell::GetFirst();
     while (pViewShell)