Fixed error when adding incoming webhook to public channel not currently in (#3483)

api/webhook.go

@@ -77,6 +77,7 @@ func createIncomingHook(c *Context, w http.ResponseWriter, r *http.Request) {
 			c.LogAudit("fail - bad channel permissions")
 			return
 		}
+		c.Err = nil
 	}
 
 	if result := <-Srv.Store.Webhook().SaveIncoming(hook); result.Err != nil {

api/webhook_test.go

@@ -18,6 +18,7 @@ func TestCreateIncomingHook(t *testing.T) {
 	team := th.SystemAdminTeam
 	channel1 := th.CreateChannel(Client, team)
 	channel2 := th.CreatePrivateChannel(Client, team)
+	channel3 := th.CreateChannel(Client, team)
 	user2 := th.CreateUser(Client)
 	LinkUserToTeam(user2, team)
 
@@ -68,6 +69,13 @@ func TestCreateIncomingHook(t *testing.T) {
 		}
 	}
 
+	Client.Must(Client.LeaveChannel(channel3.Id))
+
+	hook = &model.IncomingWebhook{ChannelId: channel3.Id, UserId: user.Id, TeamId: team.Id}
+	if _, err := Client.CreateIncomingWebhook(hook); err != nil {
+		t.Fatal(err)
+	}
+
 	Client.Logout()
 	Client.Must(Client.LoginById(user2.Id, user2.Password))
 	Client.SetTeamId(team.Id)