Refs #4333 Put back in old behaviour for original mtd.

Didn't mean to take it out and it will cause havoc at the moment if we
do

Code/Mantid/Framework/PythonAPI/MantidFramework.py

@@ -1312,13 +1312,16 @@ def _retrieveWorkspace(self, name):
         attrs = ['_getRawIPeaksWorkspacePointer', '_getRawIEventWorkspacePointer', '_getRawMatrixWorkspacePointer',
                  '_getRawIMDEventWorkspacePointer','_getRawIMDHistoWorkspacePointer',
                 '_getRawIMDWorkspacePointer', '_getRawWorkspaceGroupPointer', '_getRawTableWorkspacePointer']
-
-        for att in attrs:
-            retval = getattr(self, att)(name)
-            if retval is not None: 
-                return retval
+        retval = None
+        try:
+            for att in attrs:
+                retval = getattr(self, att)(name)
+                if retval is not None: 
+                    break
+        except RuntimeError:
+            pass
         # Preserve behaviour
-        return None
+        return retval
 
     def _workspaceRemoved(self, name):
         """