fixed crash due to the document metadata

src/document/UBDocumentProxy.cpp

@@ -297,6 +297,8 @@ QString UBDocumentProxy::sessionAuthors()
 
 QDateTime UBDocumentProxy::documentDate()
 {
+    qDebug()<< UBSettings::documentDate;
+    qDebug()<<mMetaDatas;
     if(mMetaDatas.contains(UBSettings::documentDate))
         return UBStringUtils::fromUtcIsoDate(metaData(UBSettings::documentDate).toString());
     return QDateTime::currentDateTime();

src/gui/UBTeacherGuideWidget.cpp

@@ -723,10 +723,10 @@ void UBTeacherGuidePageZeroEditionWidget::onActiveSceneChanged()
 {
     UBDocumentProxy* documentProxy = UBApplication::documentController ? UBApplication::documentController->getCurrentDocument() : 0;
     if(UBApplication::documentController && UBApplication::boardController->activeSceneIndex() == 0){
-        QDateTime creationDate = documentProxy->documentDate();
+//        QDateTime creationDate = documentProxy->documentDate();
-        mpCreationLabel->setText(tr("Created the:") + creationDate.toString(Qt::SystemLocaleShortDate));
+//        mpCreationLabel->setText(tr("Created the:") + creationDate.toString(Qt::SystemLocaleShortDate));
-        QDateTime updatedDate = documentProxy->lastUpdate();
+//        QDateTime updatedDate = documentProxy->lastUpdate();
-        mpLastModifiedLabel->setText(tr("Updated the:") + updatedDate.toString(Qt::SystemLocaleShortDate));
+//        mpLastModifiedLabel->setText(tr("Updated the:") + updatedDate.toString(Qt::SystemLocaleShortDate));
     }
 }