Fix fetching invites for a GroupChannel.

Closes #2394
Fixes #2383

discord/invite.py

@@ -229,8 +229,8 @@ class Invite(Hashable):
         How long the before the invite expires in seconds. A value of 0 indicates that it doesn't expire.
     code: :class:`str`
         The URL fragment used for the invite.
-    guild: Union[:class:`Guild`, :class:`Object`, :class:`PartialInviteGuild`]
-        The guild the invite is for.
+    guild: Optional[Union[:class:`Guild`, :class:`Object`, :class:`PartialInviteGuild`]]
+        The guild the invite is for. Can be ``None`` if it's from a group direct message.
     revoked: :class:`bool`
         Indicates if the invite has been revoked.
     created_at: :class:`datetime.datetime`
@@ -278,17 +278,28 @@ class Invite(Hashable):
 
     @classmethod
     def from_incomplete(cls, *, state, data):
+        try:
             guild_id = int(data['guild']['id'])
-        channel_id = int(data['channel']['id'])
-        guild = state._get_guild(guild_id)
-        if guild is not None:
-            channel = guild.get_channel(channel_id)
+        except KeyError:
+            # If we're here, then this is a group DM
+            guild = None
         else:
-            channel_data = data['channel']
+            guild = state._get_guild(guild_id)
+            if guild is None:
+                # If it's not cached, then it has to be a partial guild
                 guild_data = data['guild']
+                guild = PartialInviteGuild(state, guild_data, guild_id)
+
+        # As far as I know, invites always need a channel
+        # So this should never raise.
+        channel_data = data['channel']
+        channel_id = int(channel_data['id'])
         channel_type = try_enum(ChannelType, channel_data['type'])
         channel = PartialInviteChannel(id=channel_id, name=channel_data['name'], type=channel_type)
-            guild = PartialInviteGuild(state, guild_data, guild_id)
+        if guild is not None:
+            # Upgrade the partial data if applicable
+            channel = guild.get_channel(channel_id) or channel
+
         data['guild'] = guild
         data['channel'] = channel
         return cls(state=state, data=data)