fix: check for _upgrade existence

lib/charms/mongodb/v0/upgrade_helpers.py

@@ -33,7 +33,7 @@
 
 # Increment this PATCH version before using `charmcraft publish-lib` or reset
 # to 0 if you are raising the major API version
-LIBPATCH = 2
+LIBPATCH = 3
 
 logger = logging.getLogger(__name__)
 
@@ -692,7 +692,7 @@ def are_pre_upgrade_operations_config_server_successful(self):
         # again automatically on refresh (just in case the user forgot to). Disabling the balancer
         # can negatively impact the cluster, so we only disable it once the upgrade sequence has
         # begun.
-        if self._upgrade.in_progress:
+        if self._upgrade and self._upgrade.in_progress:
             try:
                 self.turn_off_and_wait_for_balancer()
             except BalancerStillRunningError: