Fix TypeError crash when using url of an invalid user

src/modules/users.js

@@ -91,7 +91,9 @@ export const getters = {
   userById: state => id =>
     state.users.find(user => user.id === id),
   userByName: state => name =>
-    state.users.find(user => user.screen_name.toLowerCase() === name.toLowerCase())
+    state.users.find(user => user.screen_name &&
+      (user.screen_name.toLowerCase() === name.toLowerCase())
+    )
 }
 
 export const defaultState = {