--- /tmp/window-manager-launcher	2015-12-27 17:25:33.090876353 +0100
+++ /usr/bin/window-manager-launcher	2015-12-27 17:31:20.629657855 +0100
@@ -33,7 +33,10 @@
         if process in pname:
             pid = int(line.split(None, 1)[0])
             print ("Killing pid %d (%s)" % (pid, pname))
-            os.kill(pid, signal.SIGKILL)
+            try:
+                os.kill(pid, signal.SIGKILL)
+            except PermissionError:
+                print ("caught PermissionError when killing old compositors/managers, continuing to allow Switch User")
 
 if wm == "marco":
     settings = Gio.Settings("org.mate.Marco.general")