Fix calls to nullable events

Tasks/UrlTester/DT2/DT2.cs

@@ -88,16 +88,16 @@ namespace DT2 {
         }
 
         public int GetUrlSync(string url) {
-            PageStart(url);
+            PageStart?.Invoke(url);
             int size = GetPageSize(url);
-            PageLoaded(url, size);
+            PageLoaded?.Invoke(url, size);
             return size;
         }
 
         public async Task<int> GetUrlAsync(string url) {
-            PageStart(url);
+            PageStart?.Invoke(url);
             int size = await Task.Run(() => GetPageSize(url));
-            PageLoaded(url, size);
+            PageLoaded?.Invoke(url, size);
             return size;
         }
 
@@ -108,7 +108,7 @@ namespace DT2 {
 
             try {
                 size = await task.WaitAsync(TimeSpan.FromMilliseconds(millis));
-                PageLoaded(url, size);
+                PageLoaded?.Invoke(url, size);
             }
             catch (TimeoutException)
             {