SdXImpressDocument::initializeForTiledRendering: missing guard

Change-Id: Idc006b63157daea880ba1eeac3832617a0992d48

sd/source/ui/unoidl/unomodel.cxx

@@ -2349,6 +2349,7 @@ Size SdXImpressDocument::getDocumentSize()
 
 void SdXImpressDocument::initializeForTiledRendering()
 {
+    SolarMutexGuard aGuard;
     // tiled rendering works only when we are in the 'Normal' view, switch to that
     mpDocShell->GetViewShell()->GetViewFrame()->GetDispatcher()->Execute(SID_VIEWSHELL0, SfxCallMode::SYNCHRON | SfxCallMode::RECORD);
 }