Add null-check for target tile in GroundMarkersPlugin (#7112)

Sometimes there is no target tile (e.g for climb-down ladders).

runelite-client/src/main/java/net/runelite/client/plugins/groundmarkers/GroundMarkerPlugin.java

@@ -286,6 +286,10 @@ public class GroundMarkerPlugin extends Plugin
 		}
 
 		Tile target = client.getSelectedSceneTile();
+		if (target == null)
+		{
+			return;
+		}
 		markTile(target.getLocalLocation());
 	}