Resolves: tdf#100191 crash: switch to Display Mode: Notes via toolbar

Change-Id: I65f5f68433940fa0b50ad951fbb96085178a42d9
diff --git a/sd/source/ui/sidebar/SlideBackground.cxx b/sd/source/ui/sidebar/SlideBackground.cxx
index 73da0e7..28e9bd9 100644
--- a/sd/source/ui/sidebar/SlideBackground.cxx
+++ b/sd/source/ui/sidebar/SlideBackground.cxx
@@ -349,7 +349,8 @@ void SlideBackground::populateMasterSlideDropdown()

void SlideBackground::updateMasterSlideSelection()
{
    SdPage* pPage = mrBase.GetMainViewShell().get()->getCurrentPage();
    ViewShell* pMainViewShell = mrBase.GetMainViewShell().get();
    SdPage* pPage = pMainViewShell ? pMainViewShell->getCurrentPage() : nullptr;
    if (pPage != nullptr && pPage->TRG_HasMasterPage())
    {
        SdrPage& rMasterPage (pPage->TRG_GetMasterPage());