Adding assertions

lib/spritesheet.dart

@@ -22,15 +22,22 @@ class SpriteSheet {
     );
   }
 
-  Sprite getSprite(int row, int column) => _sprites[row][column];
+  Sprite getSprite(int row, int column) {
+    final Sprite s = _sprites[row][column];
+
+    assert(s != null, 'No sprite found for row $row and column $column');
+
+    return s;
+  }
 
   /// Creates an animation from this SpriteSheet
   ///
   /// An [from]  and a [to]  parameter can be specified to create an animation from a subset of the columns on the row
   Animation createAnimation(int row, { double stepTime, loop = true, int from = 0, int to }) {
-    List<Sprite> spriteRow = _sprites[row];
+    final spriteRow = _sprites[row];
+
+    assert(spriteRow != null, 'There is no row for $row index');
 
-    if (spriteRow != null) {
     to ??= spriteRow.length;
 
     final spriteList = spriteRow.sublist(from, to);
@@ -41,8 +48,5 @@ class SpriteSheet {
         loop: loop,
     );
   }
-
-    return null;
-  }
 }