WPF - Fix WpfIMEKeyboardHandler crash when browser is not initalized. (#3690)

GetBrowserHost will throw exception when browser is not initalized.

Author: kkwpsv

CefSharp.Wpf/Experimental/WpfIMEKeyboardHandler.cs

@@ -190,7 +190,7 @@ private IntPtr SourceHook(IntPtr hwnd, int msg, IntPtr wParam, IntPtr lParam, re
         {
             handled = false;
 
-            if (!isActive || !isSetup || owner == null || owner.IsDisposed || owner.GetBrowserHost() == null)
+            if (!isActive || !isSetup || owner == null || owner.IsDisposed || !owner.IsBrowserInitialized || owner.GetBrowserHost() == null)
             {
                 return IntPtr.Zero;
             }