maShowPageBreaksTimer.Stop() in ScGridWindow::dispose(), tdf#124983 follow-up

Stop this timer when the ScGridWindow is to be deleted, together
with the underlying document. See
https://gerrit.libreoffice.org/c/core/+/90101
comments from Miklos Vajna on 2021-06-29 14:08/14:10 CEST

Change-Id: Ia1f39c406d391abd78a57c1d92616dbff788ec63
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/118112
Reviewed-by: Eike Rathke <erack@redhat.com>
Tested-by: Jenkins
diff --git a/sc/source/ui/view/gridwin.cxx b/sc/source/ui/view/gridwin.cxx
index ec90a6b..ab05a86 100644
--- a/sc/source/ui/view/gridwin.cxx
+++ b/sc/source/ui/view/gridwin.cxx
@@ -437,6 +437,8 @@ ScGridWindow::~ScGridWindow()

void ScGridWindow::dispose()
{
    maShowPageBreaksTimer.Stop();

    ImpDestroyOverlayObjects();

    mpFilterBox.reset();