Prevents sprite update methods to be called before the sprite box has been property intialized

packages/flutter_sprites/lib/sprite_box.dart

diff --git a/packages/flutter_sprites/lib/sprite_box.dart b/packages/flutter_sprites/lib/sprite_box.dart
index 3abf3be..90f88c3 100644
--- a/packages/flutter_sprites/lib/sprite_box.dart
+++ b/packages/flutter_sprites/lib/sprite_box.dart
@@ -86,6 +86,8 @@
     return _visibleArea;
   }
 
+  bool _initialized = false;
+
   // Setup
 
   /// Creates a new SpriteBox with a node as its content, by default uses letterboxing.
@@ -136,6 +138,7 @@
     size = constraints.biggest;
     _invalidateTransformMatrix();
     _callSpriteBoxPerformedLayout(_rootNode);
+    _initialized = true;
   }
 
   // Adding and removing nodes
@@ -361,11 +364,13 @@
 
     _frameRate = 1.0/delta;
 
-    _callConstraintsPreUpdate(delta);
-    _runActions(delta);
-    _callUpdate(_rootNode, delta);
-    _callStepPhysics(delta);
-    _callConstraintsConstrain(delta);
+    if (_initialized) {
+      _callConstraintsPreUpdate(delta);
+      _runActions(delta);
+      _callUpdate(_rootNode, delta);
+      _callStepPhysics(delta);
+      _callConstraintsConstrain(delta);
+    }
 
     // Schedule next update
     _scheduleTick();