STemWin with keil rtx

Hi all, i'm trying to use emWin librarie with keil rtos and i have some problems. First, i use a STM32F7 discovery with a touch screen. For my project, i started with the GUIDemo (keil package exemple). I remove all the exemple and build a new one adapt for me. It work fine if i let GUI_Exec() in an infinite loop (in the main). Below the working code :

int main (void) {

  CPU_CACHE_Enable();                       /* Enable the CPU Cache           */
  HAL_Init();                               /* Initialize the HAL Library     */
  BSP_SDRAM_Init();                         /* Initialize BSP SDRAM           */
  SystemClock_Config();                     /* Configure the System Clock     */
  MainTask();
  GUI_Exec();
  for (;;);

}

Now, if i try to use thread (semaphore), i'm doing this :

int main (void) {

  CPU_CACHE_Enable();                       /* Enable the CPU Cache           */
  HAL_Init();                               /* Initialize the HAL Library     */
  BSP_SDRAM_Init();                         /* Initialize BSP SDRAM           */
  SystemClock_Config();                     /* Configure the System Clock     */
        MainTask();
        GUI_Exec();                   //need to let one guiExec here else i have a black screen
        osKernelInitialize ();
        Init_Semaphore ();
        osKernelStart ();

}

the semaphore :

#include <cmsis_os.h>                                           // CMSIS RTOS header file
#include "GUI.h"

/*----------------------------------------------------------------------------
 *      Semaphore creation & usage
 *---------------------------------------------------------------------------*/

void Thread_Semaphore (void const *argument);                   // thread function
osThreadId tid_Thread_Semaphore;                                // thread id
osThreadDef (Thread_Semaphore, osPriorityNormal, 1, 0);         // thread object

osSemaphoreId sid_Thread_Semaphore;                             // semaphore id
osSemaphoreDef (SampleSemaphore);                               // semaphore object


int Init_Semaphore (void) {

  sid_Thread_Semaphore = osSemaphoreCreate(osSemaphore(SampleSemaphore), 1);
  if (!sid_Thread_Semaphore)  {
    ; // Semaphore object not created, handle failure
  }

  tid_Thread_Semaphore  = osThreadCreate (osThread(Thread_Semaphore), NULL);
  if(!tid_Thread_Semaphore) return(-1);

  return(0);
}

void Thread_Semaphore(void const *argument) {
  int32_t val;

  while(1) {
    GUI_Exec(); // Insert thread code here...

    val = osSemaphoreWait (sid_Thread_Semaphore, 10);           // wait 10 mSec
    switch (val) {
      case osOK:
         // Use protected code here...
        osSemaphoreRelease (sid_Thread_Semaphore);              // Return a token back to a semaphore
        break;
      case osErrorResource:
        break;
      case osErrorParameter:
        break;
      default:
        break;
    }

    osThreadYield();        // suspend thread
  }
}

The application start but when i touch the screen, the application freeze and i have an os_error (not identify) ! Ihave tried with thread but same result ! For information in GUIConf ihave :

#define GUI_OS                    1

Do you know how i can resolve this ?