Don't try logging out when already logged out -- about:weave calls logout which tries to _log, but Fennec weave hasn't loaded Weave yet.

services/sync/modules/service.js

@@ -743,6 +743,10 @@ WeaveSvc.prototype = {
     })))(),
 
   logout: function WeaveSvc_logout() {
+    // No need to do anything if we're already logged out
+    if (!this._loggedIn)
+      return;
+
     this._log.info("Logging out");
     this._loggedIn = false;
     this._keyPair = {};