networkmanager: Fix JavaScript error when applying settings
networkmanager: Fix JavaScript error when applying settings

show_modal_dialog expects an action to be a function which returns a
promise, as it didn't you would get an error that undefined has no
method 'then'. The issue was introduced in 3899b9b5d79df.

mvollmer/cockpit,cockpit-project/cockpit,martinpitt/cockpit,mvollmer/cockpit,martinpitt/cockpit,cockpit-project/cockpit,mvollmer/cockpit,mvollmer/cockpit,cockpit-project/cockpit,martinpitt/cockpit,martinpitt/cockpit,cockpit-project/cockpit,martinpitt/cockpit,cockpit-project/cockpit,mvollmer/cockpit