reset the label if null string was previous value (#1128)

plugins/org.yakindu.sct.ui.editor/src/org/yakindu/sct/ui/editor/editparts/PlugableExternalXtextLabelEditPart.java

@@ -140,7 +140,7 @@ public abstract class PlugableExternalXtextLabelEditPart extends ExternalXtextLa
 	@Override
 	protected void updateLabelText() {
 		String label = (String) resolveSemanticElement().eGet(getAttribute());
-		getFigure().setText(label);
+		getFigure().setText(label == null ? "" : label);
 	}
 	
 	@Override