Fix multi-guild nickname channel names.

CHANGELOG.md

@@ -6,6 +6,11 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 This project mostly adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html);
 however, insignificant breaking changes do not guarantee a major version bump, see the reasoning [here](https://github.com/modmail-dev/modmail/issues/319). If you're a plugin developer, note the "BREAKING" section.
 
+# v4.1.3
+
+### Fixed
+- `use_nickname_channel_name` option would produce errors when the member was not found in the guild leading to the failure of opening modmail threads. ([PR #3368](https://github.com/modmail-dev/Modmail/pull/3368))
+
 # v4.1.2
 
 ### Fixed

bot.py

@@ -1752,31 +1752,37 @@ def format_channel_name(self, author, exclude_channel=None, force_null=False):
         guild = self.modmail_guild
 
         if force_null:
-            name = new_name = "null"
+            return ensure_unique_channel_name("null", guild, exclude_channel)
         else:
             if self.config["use_random_channel_name"]:
                 to_hash = self.token.split(".")[-1] + str(author.id)
                 digest = hashlib.md5(to_hash.encode("utf8"), usedforsecurity=False)
-                name = new_name = digest.hexdigest()[-8:]
+                name = digest.hexdigest()[-8:]
             elif self.config["use_user_id_channel_name"]:
-                name = new_name = str(author.id)
+                name = str(author.id)
             elif self.config["use_timestamp_channel_name"]:
-                name = new_name = author.created_at.isoformat(sep="-", timespec="minutes")
+                name = author.created_at.isoformat(sep="-", timespec="minutes")
             else:
                 if self.config["use_nickname_channel_name"]:
                     author_member = self.guild.get_member(author.id)
-                    name = author_member.display_name.lower()
+                    if author_member is None:
+                        name = author.display_name.lower()
+                    else:
+                        name = author_member.display_name.lower()
                 else:
                     name = author.name.lower()
 
-                if force_null:
-                    name = "null"
+        sanitized_name = self.sanitize_name(name)
+
+        if author.discriminator != "0":
+            sanitized_name += f"-{author.discriminator}"
+
+        return ensure_unique_channel_name(sanitized_name, guild, exclude_channel)
 
-                name = "".join(l for l in name if l not in string.punctuation and l.isprintable()) or "null"
-                if author.discriminator != "0":
-                    name += f"-{author.discriminator}"
-                new_name = name
+    def sanitize_name(self, name: str) -> str:
+        return "".join(l for l in name if l not in string.punctuation and l.isprintable()) or "null"
 
+    def ensure_unique_channel_name(self, name, guild, exclude_channel) -> str:
         counter = 1
         existed = set(c.name for c in guild.text_channels if c != exclude_channel)
         while new_name in existed: