Refs #8349. Quick fix for the problem I've found.

Don't show fitting results in the workspace selector on DA tab. When
fitting is done, they do get deleted before creating a new ones. Property
browser is subscribed to deletion events but not to addition events, which
causes problems in that case.

Code/Mantid/MantidQt/MantidWidgets/src/MuonFitPropertyBrowser.cpp

@@ -340,9 +340,14 @@ void MuonFitPropertyBrowser::setFitEnabled(bool yes)
 bool MuonFitPropertyBrowser::isWorkspaceValid(Mantid::API::Workspace_sptr ws)const
 {
   QString workspaceName(QString::fromStdString(ws->name() ) );
+
   if ( (workspaceName.contains("_Raw") ) || (workspaceName.contains("MuonAnalysis") ) )
     return false;
 
+  // Exclude fitting results
+  if ( workspaceName.endsWith("_Workspace") )
+    return false;
+
   if (dynamic_cast<Mantid::API::MatrixWorkspace*>(ws.get()) != 0)
     return true;
   else