Fix a very rare crash when rendering the debug F3 screen.

Author: Darkhax

src/main/java/net/darkhax/gamestages/GameStages.java

@@ -49,7 +49,7 @@ private void onF3Text (CustomizeGuiOverlayEvent.DebugText event) {
 
         if (mc.options.renderDebug) {
 
-            if (mc.player.isShiftKeyDown()) {
+            if (mc.player != null && mc.player.isShiftKeyDown()) {
 
                 final IStageData data = GameStageHelper.getPlayerData(mc.player);