RTOS: EK-TM4C1294XL, UART Thread not working

Tool/software: TI-RTOS

Hello,

I have an RTOS Project on TM4C1294XL which is currently being used to receive and transmit data to a UART on an external sensor.  There is only one thread running on this project: a Timer Module whose ISR is the UART receive/transmit function.  There is nothing else going on.  The CPU is set to 40 Mhz, and the timer module is called every 500,000 microseconds, or every 0.5 seconds.  It seems I get a different error every time, but most recently it is: ti.sysbios.gates.GateMutex: line 99: assertion failure: A_badContext: bad calling context. See GateMutex API doc for details.
xdc.runtime.Error.raise: terminating execution

I have seen other threads which ask about this error and the solution is to change the thread to a task with a semaphore pend.  But I really do not want to do this.  I simply want the Timer module to do its job.  I do not know why I would need to block anything with a semaphore when this is the ONLY thread in the system.  Code is below.

//----------------------------------------
// BIOS header files
//----------------------------------------
#include <xdc/std.h>                        //mandatory - have to include first, for BIOS types
#include <ti/sysbios/BIOS.h>                //mandatory - if you call APIs like BIOS_start()
#include <xdc/runtime/Log.h>                //needed for any Log_info() call
#include <xdc/cfg/global.h>                 //header file for statically defined objects/handles
#include <xdc/runtime/System.h>


//------------------------------------------
// TivaWare Header Files
//------------------------------------------
#include <stdint.h>
#include <stdbool.h>
#include "inc/hw_types.h"
#include "inc/hw_memmap.h"
#include "driverlib/sysctl.h"
#include "driverlib/gpio.h"
#include "inc/hw_ints.h"
#include "driverlib/interrupt.h"
#include "driverlib/timer.h"
#include "driverlib/uart.h"
#include "Board.h"
#include "driverlib/pin_map.h"

//----------------------------------------
// Definitions
//----------------------------------------

#define UART2_BAUDRATE      9600

//----------------------------------------
// Prototypes
//----------------------------------------
void hardware_init(void);
void UART_exchange(void);


//---------------------------------------
// Globals
//---------------------------------------

//Array which contains the "Read CO2 concentration" command.  This is sent from
//the target UART to the CO2 Sensor UART, to signal we want the sensor to send us
//its current CO2 reading.
uint8_t read_bytes[9] = {0xFF, 0x01, 0x86, 0x00, 0x00, 0x00, 0x00, 0x00, 0x79};

//Array which contains the Sensor Tx response.
uint8_t response_bytes[9] = {0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00};

//Holds calculated value of CO2 Concetration
uint32_t CO2_Value = 0;

//Current clock Rate
uint32_t sys_clock = 40000000;

//---------------------------------------------------------------------------
// main()
//---------------------------------------------------------------------------
void main(void)

{
   hardware_init();                         // init hardware via TIVAware
   BIOS_start();

}


void hardware_init(void)
{

    //Set CPU Clock to 40MHz. 400MHz PLL/2 = 200 DIV 5 = 40MHz
    //SysCtlClockSet(SYSCTL_SYSDIV_5|SYSCTL_USE_PLL|SYSCTL_XTAL_16MHZ|SYSCTL_OSC_MAIN);

    SysCtlPeripheralEnable(SYSCTL_PERIPH_GPIOA);
    SysCtlPeripheralEnable(SYSCTL_PERIPH_UART2);
    GPIOPinConfigure(GPIO_PA6_U2RX);
    GPIOPinConfigure(GPIO_PA7_U2TX);
    GPIOPinTypeUART(GPIO_PORTA_BASE, GPIO_PIN_6 | GPIO_PIN_7);

    UARTEnable(UART2_BASE);
    UARTConfigSetExpClk(UART2_BASE, sys_clock, UART2_BAUDRATE,(UART_CONFIG_WLEN_8 | UART_CONFIG_STOP_ONE | UART_CONFIG_PAR_NONE));

}


//---------------------------------------------------------------------------
// UART_exchange()
//
// Sends command to Sensor Rx, waits for response, stores response
// in response_bytes[9] array.
//---------------------------------------------------------------------------
void UART_exchange(void)
{
    TimerIntClear(TIMER2_BASE, TIMER_TIMA_TIMEOUT);         // must clear timer flag FROM timer

    uint32_t w,x = 0;

    for(w = 0; w < 9; w++)
    {
        UARTCharPut(UART2_BASE, read_bytes[w]);
    }

    while(UARTBusy(UART2_BASE))
    {
        //spin here until UART is done transmitting
    }

    System_printf("UART is done transmitting.\n");

    //Get the bytes in the receive FIFO

    while(UARTCharsAvail(UART2_BASE))
    {
        response_bytes[x] = UARTCharGet(UART2_BASE);
        x++;
    }

    System_printf("UART is done receiving. \n");


    CO2_Value = response_bytes[2]*256 + response_bytes[3];

    System_printf("CO2 Value in ppm is: ");
    System_printf("%d \n", CO2_Value);

}