Object reference not set to an instance of an object when close with two cefsharp.winforms instances

My wpf uses two ChromiumWebBrowser(Cefsharp.Winforms) instances, one is for my website, one is for displaying pdf file, when I close the MianWindow, there is a crash like below:

System.NullReferenceException
  HResult=0x80004003
  Message=Object reference not set to an instance of an object.
  Source=CefSharp.WinForms
  StackTrace:
   在 CefSharp.WinForms.ChromiumWebBrowser.InternalDispose(Boolean disposing)
   在 CefSharp.WinForms.ChromiumWebBrowser.Dispose(Boolean disposing)
   在 System.ComponentModel.Component.Dispose()
   在 System.Windows.Forms.Control.Dispose(Boolean disposing)
   在 System.Windows.Forms.Integration.WinFormsAdapter.Dispose(Boolean disposing)
   在 System.ComponentModel.Component.Dispose()
   在 System.Windows.Forms.Integration.WindowsFormsHost.Dispose(Boolean disposing)
   在 System.Windows.Interop.HwndHost.WeakEventDispatcherShutdown.OnShutdownFinished(Object sender, EventArgs e)
   在 System.EventHandler.Invoke(Object sender, EventArgs e)
   在 System.Windows.Threading.Dispatcher.ShutdownImplInSecurityContext(Object state)
   在 System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   在 System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   在 System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
   在 MS.Internal.CulturePreservingExecutionContext.Run(CulturePreservingExecutionContext executionContext, ContextCallback callback, Object state)
   在 System.Windows.Threading.Dispatcher.ShutdownImpl()
   在 System.Windows.Threading.Dispatcher.PushFrameImpl(DispatcherFrame frame)
   在 System.Windows.Threading.Dispatcher.PushFrame(DispatcherFrame frame)
   在 System.Windows.Application.RunDispatcher(Object ignore)
   在 System.Windows.Application.RunInternal(Window window)
   在 System.Windows.Application.Run(Window window)
   在 System.Windows.Application.Run()

I didn't set CefSharpSettings.ShutdownOnExit explicitly, in my close function, just call:

Application.Current.Shutdown();

It is ok if remove one for pdf. I host the browser in like below:

        <WindowsFormsHost Name="browserHost">
            <cefwpf:ChromiumWebBrowser x:Name="browser" IsBrowserInitializedChanged="browser_IsBrowserInitializedChanged">
            </cefwpf:ChromiumWebBrowser>
        </WindowsFormsHost>

My cefsharp version is 83.1.100 Thanks for any help!