Update django_mongodb_backend/schema.py

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: aclark4life

django_mongodb_backend/schema.py

@@ -461,6 +461,10 @@ def _get_encrypted_fields_map(self, model, client, create_data_keys=False):
         auto_encryption_opts = options.auto_encryption_opts
         kms_provider = router.kms_provider(model)
         master_key = self.connection.settings_dict.get("KMS_CREDENTIALS", {}).get(kms_provider)
+        if master_key is None:
+            raise ImproperlyConfigured(
+                f"KMS_CREDENTIALS for provider '{kms_provider}' not found in DATABASES['{self.connection.alias}']['KMS_CREDENTIALS']."
+            )
         client_encryption = ClientEncryption(
             auto_encryption_opts._kms_providers,
             auto_encryption_opts._key_vault_namespace,