message: fix(frontend): handles null users in avatar display
diff: <div style="display: inline-block">
  <v-menu v-if="loggedIn" offset-x open-on-hover>
  <template #activator="{ on, attrs }">
  - <v-avatar class="ma-1" color="grey lighten-3" :size="size" v-bind="attrs" v-on="on">
  + <v-avatar
  + v-if="user"
  + class="ma-1"
  + color="grey lighten-3"
  + :size="size"
  + v-bind="attrs"
  + v-on="on"
  + >
  <v-img v-if="avatar" :src="avatar" />
  <v-img v-else :src="`https://robohash.org/` + id + `.png?size=40x40`" />
  </v-avatar>
  + <v-avatar v-else class="ma-1" :size="size" v-bind="attrs" v-on="on">
  + <v-img contain src="/logo.svg"></v-img>
  + </v-avatar>
  </template>
  - <v-card style="width: 200px" :to="isSelf ? '/profile' : '/profile/' + id">
  + <v-card v-if="user" style="width: 200px" :to="isSelf ? '/profile' : '/profile/' + id">
  <v-card-text v-if="!$apollo.loading" class="text-center">
  <v-avatar class="my-4" color="grey lighten-3" :size="40">
  <v-img v-if="avatar" :src="avatar" />
  <br />
  </v-card-text>
  </v-card>
  + <v-card v-else>
  + <v-card-text class="text-xs">
  + <b>Speckle Robo Ghost</b>
  + <br />
  + This user no longer exists.
  + </v-card-text>
  + </v-card>
  </v-menu>
  <v-avatar v-else class="ma-1" color="grey lighten-3" :size="size">
  <v-img v-if="avatar" :src="avatar" />
  