I learned according to the official recommended documents. However, this kind of thing will happen at this step. How can I solve it?
It looks like you are calling it on a null object creating the exception.
Could you repro in the playground to confirm ? You should only call this on an existing instance.
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Also to reinforce this you may need to call some awaits or callback to confirm your mesh has loaded.
Otherwise, you will very likely get a null.
I haven’t used the playground, how to use the code of multiple files on the playground
How to call wait or callback to confirm that my grid has been loaded? I read the tutorial on the official web page. This situation does not appear in it
return SceneLoader.ImportMeshAsync(null, “./models/”, “player.glb”, scene).then((result) =>{
The then part is the callback once your mesh has been loaded. (unless you are talking about something else?) As for merging multiple files for a playground, you can create different classes and functions all in one file. So you are not going to need to import anything from across files. 
OK, OK, I see. I’ve shown it. But now there is a problem. When I first entered the game, the keyboard keys did not respond. I need to move the cursor out of the canvas, click (red arrow), and then click back into the canvas (green arrow). The keyboard keys can be triggered.
