Fix undefined exception in the channelSettings flex-tab

In the channel settings flex-tab, there is an exception in template helper: TypeError: Cannot read property 'get' of undefined, when read only is toggled.

packages/rocketchat-channel-settings/client/views/channelSettings.coffee

@@ -195,11 +195,14 @@ Template.channelSettings.onCreated ->
 		reactWhenReadOnly:
 			type: 'boolean'
 			label: 'React_when_read_only'
+			processing: new ReactiveVar(false)
 			canView: (room) => room.t isnt 'd' and room.ro
 			canEdit: (room) => RocketChat.authz.hasAllPermission('set-react-when-readonly', room._id)
 			save: (value, room) ->
+				@processing.set(true)
 				Meteor.call 'saveRoomSettings', room._id, 'reactWhenReadOnly', value, (err, result) ->
 					return handleError err if err
+					@processing.set(false)
 					toastr.success TAPi18n.__ 'React_when_read_only_changed_successfully'
 
 		archived: