[commands] Fix guild channel converters to work in DMs

Fix #7147

discord/ext/commands/converter.py

@@ -393,10 +393,10 @@ class GuildChannelConverter(IDConverter[discord.abc.GuildChannel]):
     """
 
     async def convert(self, ctx: Context, argument: str) -> discord.abc.GuildChannel:
-        return self._resolve_channel(ctx, argument, ctx.guild.channels, discord.abc.GuildChannel)
+        return self._resolve_channel(ctx, argument, 'channels', discord.abc.GuildChannel)
 
     @staticmethod
-    def _resolve_channel(ctx: Context, argument: str, iterable: Iterable[CT], type: Type[CT]) -> CT:
+    def _resolve_channel(ctx: Context, argument: str, attribute: str, type: Type[CT]) -> CT:
         bot = ctx.bot
 
         match = IDConverter._get_id_match(argument) or re.match(r'<#([0-9]{15,20})>$', argument)
@@ -406,6 +406,7 @@ class GuildChannelConverter(IDConverter[discord.abc.GuildChannel]):
         if match is None:
             # not a mention
             if guild:
+                iterable: Iterable[CT] = getattr(guild, attribute)
                 result: Optional[CT] = discord.utils.get(iterable, name=argument)
             else:
 
@@ -443,7 +444,7 @@ class TextChannelConverter(IDConverter[discord.TextChannel]):
     """
 
     async def convert(self, ctx: Context, argument: str) -> discord.TextChannel:
-        return GuildChannelConverter._resolve_channel(ctx, argument, ctx.guild.text_channels, discord.TextChannel)
+        return GuildChannelConverter._resolve_channel(ctx, argument, 'text_channels', discord.TextChannel)
 
 
 class VoiceChannelConverter(IDConverter[discord.VoiceChannel]):
@@ -463,7 +464,7 @@ class VoiceChannelConverter(IDConverter[discord.VoiceChannel]):
     """
 
     async def convert(self, ctx: Context, argument: str) -> discord.VoiceChannel:
-        return GuildChannelConverter._resolve_channel(ctx, argument, ctx.guild.voice_channels, discord.VoiceChannel)
+        return GuildChannelConverter._resolve_channel(ctx, argument, 'voice_channels', discord.VoiceChannel)
 
 
 class StageChannelConverter(IDConverter[discord.StageChannel]):
@@ -482,7 +483,7 @@ class StageChannelConverter(IDConverter[discord.StageChannel]):
     """
 
     async def convert(self, ctx: Context, argument: str) -> discord.StageChannel:
-        return GuildChannelConverter._resolve_channel(ctx, argument, ctx.guild.stage_channels, discord.StageChannel)
+        return GuildChannelConverter._resolve_channel(ctx, argument, 'stage_channels', discord.StageChannel)
 
 
 class CategoryChannelConverter(IDConverter[discord.CategoryChannel]):
@@ -502,7 +503,7 @@ class CategoryChannelConverter(IDConverter[discord.CategoryChannel]):
     """
 
     async def convert(self, ctx: Context, argument: str) -> discord.CategoryChannel:
-        return GuildChannelConverter._resolve_channel(ctx, argument, ctx.guild.categories, discord.CategoryChannel)
+        return GuildChannelConverter._resolve_channel(ctx, argument, 'categories', discord.CategoryChannel)
 
 
 class StoreChannelConverter(IDConverter[discord.StoreChannel]):
@@ -521,7 +522,7 @@ class StoreChannelConverter(IDConverter[discord.StoreChannel]):
     """
 
     async def convert(self, ctx: Context, argument: str) -> discord.StoreChannel:
-        return GuildChannelConverter._resolve_channel(ctx, argument, ctx.guild.channels, discord.StoreChannel)
+        return GuildChannelConverter._resolve_channel(ctx, argument, 'channels', discord.StoreChannel)
 
 
 class ColourConverter(Converter[discord.Colour]):