don't crash on race with room members and initial composer render

not ideal, but for now this prevents a crash at startup
when a user-pill is persisted in local storage

src/editor/parts.js

@@ -284,6 +284,9 @@ class UserPillPart extends PillPart {
     }
 
     setAvatar(node) {
+        if (!this._member) {
+            return;
+        }
         const name = this._member.name || this._member.userId;
         const defaultAvatarUrl = Avatar.defaultAvatarUrlForString(this._member.userId);
         let avatarUrl = Avatar.avatarUrlForMember(