TelegramClient: Remove redundant check for adding update handlers

Adding update handlers when not authorized makes no negative consequences.
Also, check was implemented badly - update handlers can be added after logout/disconnect, for example.

telethon/telegram_client.py

@@ -79,9 +79,6 @@ class TelegramClient:
         self.sender = None
         self.phone_code_hashes = {}
 
-        # We need to be signed in before we can listen for updates
-        self.signed_in = False
-
     # endregion
 
     # region Connecting
@@ -121,8 +118,6 @@ class TelegramClient:
             # although many other options are available!
             self.dc_options = result.dc_options
 
-            # We're signed in if we're authorized
-            self.signed_in = self.is_user_authorized()
             return True
         except RPCError as error:
             print('Could not stabilise initial connection: {}'.format(error))
@@ -241,9 +236,6 @@ class TelegramClient:
         self.session.user = result.user
         self.session.save()
 
-        # Now that we're authorized, set the signed_in flag
-        # to True so update handlers can be added
-        self.signed_in = True
         return True
 
     def sign_up(self, phone_number, code, first_name, last_name=''):
@@ -728,9 +720,9 @@ class TelegramClient:
     def add_update_handler(self, handler):
         """Adds an update handler (a function which takes a TLObject,
           an update, as its parameter) and listens for updates"""
-        if not self.signed_in:
-            raise ValueError(
-                "You cannot add update handlers until you've signed in.")
+        if not self.sender:
+            raise RuntimeError(
+                "You should connect at least once to add update handlers.")
 
         self.sender.add_update_handler(handler)