when the button is clicked, it is located on a fragment, and the method from MainActivity is called. after selecting from the dialog box, the method in the fragment is called again. But in this fragment, binding becomes null and (activity as MainActivity) is also null.

Fragment:

class HomeFragment :  Fragment(){
    private lateinit var binding: FragmentHomeBinding
    private lateinit var mainActivity: MainActivity

    override fun onCreateView(
        inflater: LayoutInflater, container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View {
        binding = FragmentHomeBinding.inflate(inflater)
        return binding.root
    }

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)
        mainActivity = (activity as MainActivity)
        binding.addButton.setOnClickListener{
            mainActivity.showDialog()
        }
    }

    fun getText(newText: TextView){
        binding.layout.addView(newText)
    }
}

MainActivity:

class MainActivity : AppCompatActivity() {
    private lateinit var binding: ActivityMainBinding
    private lateinit var navController: NavController

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        binding = ActivityMainBinding.inflate(layoutInflater)
        setContentView(binding.root)

        val navHostFragment = 
        supportFragmentManager.findFragmentById(R.id.mainContainer) as NavHostFragment
        navController = navHostFragment.navController
        val bottomNavigationView = binding.bottomNavigationView
        setupWithNavController(bottomNavigationView,navController)
    }

    private fun createText(text: String, color: Int): TextView {
        val textView = TextView(this)
        textView.setBackgroundColor(color)
        textView.text = text
        textView.setLayoutParams(
            LinearLayout.LayoutParams(
                LinearLayout.LayoutParams.WRAP_CONTENT,
                LinearLayout.LayoutParams.WRAP_CONTENT
            )
        )

        return textView
    }

    fun showDialog(){
        val langArray: Array<String> = arrayOf("Задача", "Событие")
        val selectedEvent = BooleanArray(langArray.size)
        val langList: ArrayList<Int> = ArrayList()
        val builder: AlertDialog.Builder = AlertDialog.Builder(this)

        builder.setMultiChoiceItems(langArray, selectedEvent
        ) { dialogInterface, i, b ->
            if (b) {
                langList.add(i)
                langList.sort()
            } else {
                langList.remove(i)
            }
        }

        builder.setPositiveButton("OK"
        ) { dialogInterface, i -> // Initialize string builder
            if (langList.size != 1) {
                val stringBuilder = StringBuilder()
                for (j in langList.indices) {
                    stringBuilder.append(langArray[langList[j]])
                    if (j != langList.size - 1) {
                        stringBuilder.append(", ")
                    }
                }
                Toast.makeText(
                    applicationContext,
                    "Error!!!",
                    Toast.LENGTH_LONG
                ).show()
            } else {
                val mFragment = HomeFragment()

                if (langList[0] == 0) {
                } else {
                    mFragment.getText(createText("HELLO!", R.color.green))
                }
            }
        }

        builder.setNegativeButton("Cancel"
        ) { dialogInterface, i -> // dismiss dialog
            dialogInterface.dismiss()
        }

        builder.show()
    }
}

Why is binding null?