[UWP] fix NRE in SearchBarRenderer (#2717)

- fixes #2633

Xamarin.Forms.Platform.UAP/SearchBarRenderer.cs

@@ -96,8 +96,9 @@ namespace Xamarin.Forms.Platform.UWP
 
 			// If the Forms VisualStateManager is in play or the user wants to disable the Forms legacy
 			// color stuff, then the underlying textbox should just use the Forms VSM states
-			_queryTextBox.UseFormsVsm = Element.HasVisualStateGroups()
-							|| !Element.OnThisPlatform().GetIsLegacyColorModeEnabled();
+			if (_queryTextBox != null)
+				_queryTextBox.UseFormsVsm = Element.HasVisualStateGroups()
+								|| !Element.OnThisPlatform().GetIsLegacyColorModeEnabled();
 		}
 
 		void OnQuerySubmitted(AutoSuggestBox sender, AutoSuggestBoxQuerySubmittedEventArgs e)