Verifying if GestureBinding is ready and if not, throwing a meaningful error

doc/input.md

@@ -34,4 +34,5 @@ Here are some example of more complex Gesture Recognizers:
         return new TapGestureRecognizer()
             ..onTapUp = (TapUpDetails details) => this.handleTap(details.globalPosition);;
     }
-```
+```
+__ATTENTION:__ `Flame.util.addGestureRecognizer` must be called after the `runApp`, otherwise Flutter's `GestureBinding` will not be initialized yet and exceptions will occur.

lib/util.dart

@@ -56,6 +56,10 @@ class Util {
   ///
   /// Use this in order to get it to work in case your app also contains other widgets.
   void addGestureRecognizer(GestureRecognizer recognizer) {
+    if (GestureBinding.instance == null) {
+      throw new Exception("GestureBinding is not initialized yet, this probably happened because addGestureRecognizer was called before the runApp method");
+    }
+
     GestureBinding.instance.pointerRouter.addGlobalRoute((PointerEvent e) {
       if (e is PointerDownEvent) {
         recognizer.addPointer(e);