DataFromData guard against empty file item

GUI/Model/Job/DataSource.cpp

@@ -73,7 +73,9 @@ Data2DItem* DataSource::currentData2DItem() const
 
 DataItem* DataFromData::realDataItem() const
 {
-    return gDoc->datafilesRW()->currentItem()->dataItem();
+    if (DatafileItem* dfi = gDoc->datafilesRW()->currentItem())
+	return dfi->dataItem();
+    return nullptr;
 }
 
 Data1DItem* DataFromData::realData1DItem() const