I don't think the error is the bridge. If the bridge is not there, the error is "Cannot find device "br-ex"'.
I could reproduce the error by passing the argument with quotes
# ip 'route 0.0.0.0/0 via 10.0.0.1 dev br-ex'
Object "route 0.0.0.0/0 via 10.0.0.1 dev br-ex" is unknown, try "ip help".
so it's like we are passing the arguments in the wrong way
I don't think the error is the bridge. If the bridge is not there, the error is "Cannot find device "br-ex"'.
I could reproduce the error by passing the argument with quotes
# ip 'route 0.0.0.0/0 via 10.0.0.1 dev br-ex'
Object "route 0.0.0.0/0 via 10.0.0.1 dev br-ex" is unknown, try "ip help".
so it's like we are passing the arguments in the wrong way