WPF/OffScreen - NullReferenceException in the WaitForRenderIdleAsync method

- Catch and throw exception back onto the calling thread so it doesn't crash the app

Resovles #4902

Author: amaitland

CefSharp.OffScreen/ChromiumWebBrowser.cs

@@ -762,6 +762,10 @@ public async Task WaitForRenderIdleAsync(int idleTimeInMs = 500, TimeSpan? timeo
                     // the same time, the timer maybe Disposed on a different thread.
                     // https://github.com/cefsharp/CefSharp/issues/4597
                 }
+                catch(Exception ex)
+                {
+                    renderIdleTcs.TrySetException(ex);
+                }
             };
 
             idleTimer.Start();

CefSharp.Wpf/ChromiumWebBrowser.cs

@@ -2754,6 +2754,10 @@ public async Task WaitForRenderIdleAsync(int idleTimeInMs = 500, TimeSpan? timeo
                     // the same time, the timer maybe Disposed on a different thread.
                     // https://github.com/cefsharp/CefSharp/issues/4597
                 }
+                catch (Exception ex)
+                {
+                    renderIdleTcs.TrySetException(ex);
+                }
             };
 
             idleTimer.Start();