use the thumbnail proxy's getter instead of the documentController's one

src/gui/UBThumbnailWidget.cpp

@@ -858,7 +858,7 @@ void UBSceneThumbnailNavigPixmap::mousePressEvent(QGraphicsSceneMouseEvent *even
 
 void UBSceneThumbnailNavigPixmap::deletePage()
 {
-    if(UBApplication::mainWindow->yesNoQuestion(QObject::tr("Remove Page"), QObject::tr("Are you sure you want to remove 1 page from the selected document '%0'?").arg(UBApplication::documentController->selectedDocument()->metaData(UBSettings::documentName).toString()))){
+    if(UBApplication::mainWindow->yesNoQuestion(QObject::tr("Remove Page"), QObject::tr("Are you sure you want to remove 1 page from the selected document '%0'?").arg(proxy()->metaData(UBSettings::documentName).toString()))){
         UBApplication::boardController->deleteScene(sceneIndex());
     }
 }