Fix unable to add NAT cloud after configuring “allowed_interfaces” in configuration file. Fixes #2508

gns3server/compute/builtin/nodes/nat.py

@@ -37,7 +37,7 @@ class Nat(Cloud):
     def __init__(self, name, node_id, project, manager, ports=None):
 
         allowed_interfaces = Config.instance().settings.Server.allowed_interfaces
-        if allowed_interfaces:
+        if allowed_interfaces and isinstance(allowed_interfaces, str):
             allowed_interfaces = allowed_interfaces.split(',')
         if sys.platform.startswith("linux"):
             nat_interface = Config.instance().settings.Server.default_nat_interface