Fix null-type bugs (#662)

This is more for consistency with Puppeteer.

Author: kblok

lib/PuppeteerSharp/CDPSession.cs

@@ -120,11 +120,12 @@ public async Task<T> SendAsync<T>(string method, dynamic args = null)
         /// <param name="args">The method args</param>
         /// <param name="rawContent"></param>
         /// <returns>The task.</returns>
+        /// <exception cref="T:PuppeteerSharp.PuppeteerException"></exception>
         public async Task<dynamic> SendAsync(string method, bool rawContent, dynamic args = null)
         {
             if (Connection == null)
             {
-                throw new Exception($"Protocol error ({method}): Session closed. Most likely the {TargetType} has been closed.");
+                throw new PuppeteerException($"Protocol error ({method}): Session closed. Most likely the {TargetType} has been closed.");
             }
             var id = ++_lastId;
             var message = JsonConvert.SerializeObject(new Dictionary<string, object>
@@ -167,8 +168,15 @@ public async Task<dynamic> SendAsync(string method, bool rawContent, dynamic arg
         /// Detaches session from target. Once detached, session won't emit any events and can't be used to send messages.
         /// </summary>
         /// <returns></returns>
+        /// <exception cref="T:PuppeteerSharp.PuppeteerException"></exception>
         public Task DetachAsync()
-            => Connection.SendAsync("Target.detachFromTarget", new { sessionId = SessionId });
+        {
+            if (Connection == null)
+            {
+                throw new PuppeteerException($"Session already detached.Most likely the { TargetType } has been closed.");
+            }
+            return Connection.SendAsync("Target.detachFromTarget", new { sessionId = SessionId });
+        }
 
         internal bool HasPendingCallbacks() => _callbacks.Count != 0;
 

lib/PuppeteerSharp/ElementHandle.cs

@@ -126,7 +126,7 @@ public async Task<string> ScreenshotBase64Async(ScreenshotOptions options)
             }
 
             var viewport = Page.Viewport;
-            if (boundingBox.Width > viewport.Width || boundingBox.Height > viewport.Height)
+            if (viewport != null && (boundingBox.Width > viewport.Width || boundingBox.Height > viewport.Height))
             {
                 var newRawViewport = JObject.FromObject(viewport);
                 newRawViewport.Merge(new ViewPortOptions

lib/PuppeteerSharp/Page.cs

@@ -1674,6 +1674,10 @@ private async Task<string> PerformScreenshot(ScreenshotType type, ScreenshotOpti
 
             if (options != null && options.FullPage)
             {
+                if (Viewport == null)
+                {
+                    throw new PuppeteerException("FullPage screenshots do not work without first setting viewport.");
+                }
                 dynamic metrics = await Client.SendAsync("Page.getLayoutMetrics").ConfigureAwait(false);
                 var width = Convert.ToInt32(Math.Ceiling(Convert.ToDecimal(metrics.contentSize.width.Value)));
                 var height = Convert.ToInt32(Math.Ceiling(Convert.ToDecimal(metrics.contentSize.height.Value)));