The error "NullReferenceException: Object reference not set to an instance of an object" means that you are trying to access a member of an object that is currently set to null. In this case, it seems like the `textMesh` object is not being assigned a value before you are trying to access its `text` property.
To fix the issue, you need to make sure that `textMesh` is assigned a valid reference to a `TextMeshProUGUI` object before trying to access its properties. This can be done by assigning the reference either in the script itself or in the Unity editor.
In the script, you can assign a reference to `textMesh` using the following code:
```C#
public TextMeshProUGUI textMesh;
void Start()
{
textMesh = GetComponent<TextMeshProUGUI>();
}
```
This code will get the `TextMeshProUGUI` component attached to the same game object that this script is attached to.
Alternatively, you can also assign the reference in the Unity editor by dragging and dropping the `TextMeshProUGUI` object onto the `TextMeshProUGUI` field in the inspector.
By ensuring that `textMesh` is assigned a valid reference before trying to access its properties, you should be able to avoid the null reference exception.