iOS Question App just close itself when re-launch

I have release a B4i App in FlightTest, everything seems ok. But I found it very strange that every time after I kill the app from iOS, I cannot open the app again, it just has a white blank screen and then close itself. If I delete the app and install it again, it works again.

I wanna know how can I check which part of my code has caused this kind of error. Many many Thanks!

The app showed Checkpoint 5->Checkpoint 4->Checkpoint 3->Checkpoint 2->Checkpoint 1 in first run. Then it showed none of the checkpoint when I open the app again after I kill it. Please help!

Private Sub Application_Start (Nav As NavigationController)
NavControl = Nav
Page1.Initialize("Page1")
Page1.RootPanel.Color = Colors.White
NavControl.ShowPage(Page1)

Msgbox2("Msg", "Checkpoint 1","X",Array("X"))

SQL1.Initialize(File.DirDocuments, "MyDb.db",True)
SQL1.ExecNonQuery("CREATE TABLE IF NOT EXISTS token (token TEXT)")

Msgbox2("Msg", "Checkpoint 2","X",Array("X"))

Dim Cursor As ResultSet
Cursor = SQL1.ExecQuery("SELECT token FROM token")
Do While Cursor.NextRow
sToken = Cursor.GetInt("token")
Loop

Msgbox2("Msg", "Checkpoint 3","X",Array("X"))

NavControl.NavigationBarVisible = False
Page1.RootPanel.LoadLayout("Layout1")
'WebView1.LoadUrl("http://xxx.org/care/app")
App.RegisterUserNotifications(True, True, True)
App.RegisterForRemoteNotifications
App.ApplicationIconBadgeNumber = 0
CheckForPushMessage
WebView1.Visible = False

Msgbox2("Msg", "Checkpoint 4","X",Array("X"))

Dim job_up As HttpJob
job_up.Initialize("app_up", Me)
job_up.PostString("http://xxx.org/care/app/up.php", "")

Msgbox2("Msg", "Checkpoint 5","X",Array("X"))

End Sub