java.lang.IllegalStateException crash reports from Google Play Console

Question

I have a trouble with my Android app. Some devices which use Android 7.1 or 8.0 often report IllegatStateException crash. I have searched on Google and tried to fix it many times but no use. I cannot figure out what causes this crash. So please help me if you have time. Here is a full report:

java.lang.IllegalStateException: 

  at android.support.v4.app.FragmentManagerImpl.addFragment (FragmentManagerImpl.java:60)

  at android.support.v4.app.BackStackRecord.executeOps (BackStackRecord.java:137)

  at android.support.v4.app.FragmentManagerImpl.executeOps (FragmentManagerImpl.java:38)

  at android.support.v4.app.FragmentManagerImpl.executeOpsTogether (FragmentManagerImpl.java:112)

  at android.support.v4.app.FragmentManagerImpl.removeRedundantOperationsAndExecute (FragmentManagerImpl.java:89)

  at android.support.v4.app.FragmentManagerImpl.execPendingActions (FragmentManagerImpl.java:21)

  at android.support.v4.app.FragmentManagerImpl$1.run (FragmentManagerImpl.java:2)

  at android.os.Handler.handleCallback (Handler.java:869)

  at android.os.Handler.dispatchMessage (Handler.java:101)

  at android.os.Looper.loop (Looper.java:206)

  at android.app.ActivityThread.main (ActivityThread.java:6733)

  at java.lang.reflect.Method.invoke (Method.java)

  at com.android.internal.os.Zygote$MethodAndArgsCaller.run (Zygote.java:240)

  at com.android.internal.os.ZygoteInit.main (ZygoteInit.java:845)

I have a main activity contains a FrameLayout. I use this FrameLayout to display 5 different fragments by using:

FragmentTransaction ft = getSupportFragmentManager().beginTransaction();
switch (button) {
    case 1:
        ft.replace(R.id.mainframe, fragment1, FRAGMENT1_TAG);
        break;
    case 2:
        ft.replace(R.id.mainframe, fragment2, FRAGMENT2_TAG);
        break;
    case 4:
        Fragment fragment4 = new Fragment();
        ft.replace(R.id.mainframe, fragment4, FRAGMENT4_TAG);
        break;
}
ft.setTransition(FragmentTransaction.TRANSIT_ENTER_MASK);
ft.commit();

Some fragments must be saved to restore later (fragment 1 -> 3), some must be refreshed (create new instance) (fragment 4 -> 5). Moreover, on fragment 4 and 5, there are a viewpager and an array of button to move between pages for each. The code of viewpagers:

@Override
public void onViewCreated(@NonNull View view, @Nullable Bundle savedInstanceState) {
    super.onViewCreated(view, savedInstanceState);
    viewPager.setAdapter(new FragmentPagerAdapter(getChildFragmentManager(), new Fragment[] {
            new Page1(),
            new Page2()
    }));
}

private void onButtonTap(Button btn) {
    switch (btn.getId()) {
        case R.id.button1:
            viewPager.setCurrentItem(0, true);
            break;
        case R.id.button2:
            viewPager.setCurrentItem(1, true);
            break;
    }
}

I also define 2 classes derive from Dialog and use Toast. I cannot reproduce on simulators or my devices. It just happens on a few devices. Is there anything wrong with my code? Please help me. This crash stays for a long time, I need to get rid of it. I'm looking forward to hearing from you. Thank you very much.

Long

---

Edit 1: Here is where I call ft.replace on fragment 1

@Override
protected void onResume() {
    // Check server connection to go back to the first fragment
    if (!ConnectivityReceiver.getInstance().isServerConnected()) {
        Fragment currentFragment = getSupportFragmentManager().findFragmentById(R.id.main_frame);
        if (currentFragment != null && 
            (FRAGMENT_4_TAG.equals(currentFragment.getTag()) || FRAGMENT_5_TAG.equals(currentFragment.getTag())) && isActive()) {
            .....
            ft.replace(fragment1);
        }
    }
}

Answer 1

According to the source code, the addFragment method throws that exception with the message "Fragment already added" if you attempt to add a Fragment that is already a member of the manager.

Now, the version of the FragmentManagerImpl source code that I can see looks rather different to the stack trace, so this is a bit of a guess. But I suspect that the problem is that you may be calling ft.replace on fragment2 or fragment1 when they are already part of the layout. Under certain circumstances ....