[Android] If drawable doesn't exist don't dispose of it (#4904)

Xamarin.Forms.Platform.Android/Renderers/ShellFlyoutContentRenderer.cs

@@ -112,6 +112,7 @@ namespace Xamarin.Forms.Platform.Android
 		{
 			var drawable = await Context.GetFormsDrawable(source);
 			menuItem.SetIcon(drawable);
+			drawable?.Dispose();
 		}
 	}
 }

Xamarin.Forms.Platform.Android/Renderers/ShellItemRenderer.cs

@@ -362,7 +362,7 @@ namespace Xamarin.Forms.Platform.Android
 				return;
 			var drawable = await Context.GetFormsDrawable(source);
 			menuItem.SetIcon(drawable);
-			drawable.Dispose();
+			drawable?.Dispose();
 		}
 
 		void SetupMenu()