fix background color bug

Author: zhuxingwei

Runtime/material/bottom_navigation_bar.cs

@@ -576,6 +576,7 @@ public override Widget build(BuildContext context) {
             Color backgroundColor = null;
             switch (this.widget.type) {
                 case BottomNavigationBarType.fix:
+                    backgroundColor = this.widget.backgroundColor;
                     break;
                 case BottomNavigationBarType.shifting:
                     backgroundColor = this._backgroundColor;