DA14531 disconnect error

Hi dialog:

  I use DA14531 as peripheral device and found that my program would usually enter 

//We are not in Connected State
ASSERT_ERROR(0);

in gapc_disconnect_ind_handler at app_task.c when the central disconnects. I checked the state of app task and it's APP_DISABLED. I searched the project and didn't found where the code set the state. So what should I do to locate the problem?