ŧdf#120069: Do not crash changing background color in draw

Regression from c57f10b6,
where I disabled the master background in Impress but not in Draw,
and both share some common code.

Besides, disable master background in draw as well. Follow-up of bug
tdf#111306

Change-Id: I772382b5ed8eff3a35d001ea498810613137d172
Reviewed-on: https://gerrit.libreoffice.org/60913
Tested-by: Jenkins
Reviewed-by: Xisco Faulí <xiscofauli@libreoffice.org>

sd/source/ui/sidebar/SlideBackground.cxx

@@ -279,6 +279,17 @@ void SlideBackground::HandleContextChange(
     else if ( IsDraw() )
     {
         mpMasterLabel->SetText(SdResId(STR_MASTERPAGE_LABEL));
+
+        if (maContext == maDrawOtherContext)
+        {
+            mpFillStyle->Show();
+            mpBackgroundLabel->Show();
+        }
+        else if (maContext == maDrawMasterContext)
+        {
+            mpFillStyle->Hide();
+            mpBackgroundLabel->Hide();
+        }
     }
 }
 
@@ -286,7 +297,7 @@ void SlideBackground::Update()
 {
     eFillStyle nPos = static_cast<eFillStyle>(mpFillStyle->GetSelectedEntryPos());
 
-    if(maContext != maImpressOtherContext)
+    if(maContext != maImpressOtherContext && maContext != maDrawOtherContext)
         nPos = NONE;
 
     SfxObjectShell* pSh = SfxObjectShell::Current();