You’re using the fragmentManager of the new fragment which will be null if the fragment hasn’t been added yet, like in your case. Since you’re using ?.run, nothing happens and the method returns null.

Consider adding a fragment manager parameter to your method:

fun Fragment.navigate(fm: FragmentManager): Int? {
    return fm.run {
        beginTransaction()
            .replace(
                R.id.my_container,
                [email protected],
                [email protected]::class.simpleName
            )
            .commit() 
    }
}

And then:

TestThreeFragment
    .newInstance()
    .navigate(getParentFragmentManager())

I have created this extension function for navigating between fragments which is pretty straight forward but somehow it’s not working. it’s not doing anything and nothing changes when I click the button. I think the problem is with [email protected] argument but I don’t see why that should be a problem.

fun Fragment.navigate(): Int? {
    return fragmentManager?.run {
        beginTransaction()
            .replace(
                R.id.my_container,
                [email protected],
                [email protected]::class.simpleName
            )
            .commit() 
    }
}

and the usage is like this

class TestTwoFragment : Fragment(R.layout.fragment_test_two) {
   
    ....

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        ...

        goto_three.setOnClickListener {
            TestThreeFragment
                .newInstance()
                .navigate()
        }
    }