App.MAIN_WINDOW.DataContext = App.ViewModel is giving exception how to resolve it?

Question

I am getting error for Object reference not set to an instance of an object. Throwing unhanded exception of type system.NullreferenceException occurred in xxx.exe.

Reference code: MainWindowViewModel.cs file

namespace iWave_Configuration_Tool.VM
{
        public class MainWindowViewModel
    {
           
        public SynchronizationContext ViewContext;
        public MainWindowViewModel()
        {
            UDS_ADDRESS = new UDSSourceAddress { };
            DEVICE_INFO = new DeviceInfo { };
        }
        public UDSSourceAddress UDS_ADDRESS { get; set; }
        public DeviceInfo DEVICE_INFO { get; set; }
    }
}

port.cs file:

    if (request.COMMAND_ID == CommandDefine.CommandID.GET_UDS_SOURCE_ADDRESS_REQ);
    {                                                                             
            App.MAIN_WINDOW.DataContext = null;
            App.MAIN_WINDOW.DataContext = App.ViewModel;                                                                                                               
    }