Fix CLI nil pointer panic (#13359)

cmd/mattermost/commands/channel.go

@@ -519,6 +519,10 @@ func modifyChannelCmdF(command *cobra.Command, args []string) error {
 	}
 
 	user := getUserFromUserArg(a, username)
+	if user == nil {
+		return fmt.Errorf("Unable to find user: '%v'", username)
+	}
+
 	if _, err := a.UpdateChannelPrivacy(channel, user); err != nil {
 		return errors.Wrapf(err, "Failed to update channel ('%s') privacy", args[0])
 	}

cmd/mattermost/commands/channel_test.go

@@ -227,3 +227,30 @@ func Test_searchChannelCmdF(t *testing.T) {
 		})
 	}
 }
+
+func TestModifyChannel(t *testing.T) {
+	th := Setup().InitBasic()
+	defer th.TearDown()
+
+	channel1 := th.CreatePrivateChannel()
+	channel2 := th.CreatePrivateChannel()
+
+	th.CheckCommand(t, "channel", "modify", "--public", th.BasicTeam.Name+":"+channel1.Name, "--username", th.BasicUser2.Email)
+	res, err := th.App.Srv.Store.Channel().Get(channel1.Id, false)
+	require.Nil(t, err)
+	assert.Equal(t, model.CHANNEL_OPEN, res.Type)
+
+	// should fail because user doesn't exist
+	require.Error(t, th.RunCommand(t, "channel", "modify", "--public", th.BasicTeam.Name+":"+channel2.Name, "--username", "idonotexist"))
+
+	pchannel1 := th.CreatePublicChannel()
+	pchannel2 := th.CreatePublicChannel()
+
+	th.CheckCommand(t, "channel", "modify", "--private", th.BasicTeam.Name+":"+pchannel1.Name, "--username", th.BasicUser2.Email)
+	res, err = th.App.Srv.Store.Channel().Get(pchannel1.Id, false)
+	require.Nil(t, err)
+	assert.Equal(t, model.CHANNEL_PRIVATE, res.Type)
+
+	// should fail because user doesn't exist
+	require.Error(t, th.RunCommand(t, "channel", "modify", "--private", th.BasicTeam.Name+":"+pchannel2.Name, "--username", "idonotexist"))
+}