MemberPicker: conver nil logins into empty strings to avoid crashes

src/member/MemberPicker.js

@@ -10,6 +10,9 @@ function MemberPicker(props) {
   const setMembers = (members) => {
     const newMembers = members.map((m) => {
       m.numStr = m.num.toString();
+      if (!m.login) {
+        m.login = "";
+      }
       return m;
     });
     setStateMembers(newMembers);