Android не может найти вид по идентификатору во фрагменте

У меня есть собственный EditText MomentumEditText который у меня есть в макете в xml (fragment_edit_text.xml):

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent" >

    <com.frazerm.momentum.MomentumEditText
        xmlns:android="http://schemas.android.com/apk/res/android"
        android:id="@+id/writing_edittext"
        android:layout_width="match_parent"
        android:layout_height="match_parent" />
</RelativeLayout>

Я могу успешно раздуть и отобразить макет в фрагменте, но попытка получить MomentumEditText с помощью View.findViewById(R.id.writing_edittext) возвращает null.

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
    // Inflate the layout for this fragment
    View root = inflater.inflate(R.layout.fragment_edit_text, container, true);
    MomentumEditText editText = (MomentumEditText) root.findViewById(R.id.writing_edittext);
}

Я также попытался получить MomentumEditText из onStart(), но он все равно возвращает null.

Edit= Я попытался заменить пользовательский EditText в xml простым EditText, и findViewById() работает! здесь MomentumEditText:

package com.frazerm.momentum;

import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Paint;
import android.graphics.Rect;
import android.text.Editable;
import android.text.TextWatcher;
import android.util.AttributeSet;
import android.view.Gravity;
import android.view.MotionEvent;
import android.widget.EditText;

public class MomentumEditText extends EditText  {

boolean moveCursorDisabled = true;
boolean deleteCharsDisabled = true;

private static Paint linePaint;
private static Paint marginPaint;
MomentumEditText thisEditText = this;

Editable currentText;

public MomentumEditText(Context context, AttributeSet attrs) {
    super(context);

    setCursorVisible(true);
    this.setGravity(Gravity.TOP);
    TextWatcher inputTextWatcher = new TextWatcher() {
        CharSequence textToAdd = "";
        boolean charWasDeleted = false;
        public void beforeTextChanged(CharSequence s, int start, int count, int after)  {
            if (moveCursorDisabled) {
                if( count > after ) {
                    textToAdd = s.subSequence(start + after, start + count);
                    charWasDeleted = true;
                }
            }
        }

        public void onTextChanged(CharSequence s, int start, int before, int count) {

        }

        @Override
        public void afterTextChanged(Editable s)    {
            if (moveCursorDisabled) {
                if (charWasDeleted == true) {
                    charWasDeleted = false;
                    s.append(textToAdd);
                }
                else    {
                    thisEditText.setSelection( thisEditText.getText().length() );
                }
            }
        }

    };
    this.addTextChangedListener(inputTextWatcher);

}

//disables ability to move the cursor to other parts of the text
@Override
public boolean onTouchEvent(MotionEvent event)
{
    if (deleteCharsDisabled)    {
        final float eventX = event.getX();
        if( getSelectionStart() != eventX )
        {
            super.onTouchEvent(event);
            thisEditText.setSelection( thisEditText.getText().length() );
            return true;
        }
        return super.onTouchEvent(event);
    }
    return super.onTouchEvent(event);
}

@Override
protected void onDraw(Canvas canvas) {
    linePaint = new Paint();
    linePaint.setColor(0x77777777);
    //linePaint.setStyle(Style.STROKE);

    marginPaint = new Paint();
    marginPaint.setColor(0x99FF4444);
    //marginPaint.setStyle(Style.STROKE);

    Rect bounds = new Rect();
    int firstLineY = getLineBounds(0, bounds);
    int lineHeight = getLineHeight();
    int totalLines = Math.max(getLineCount(), getHeight() / lineHeight);

    for (int i = 0; i < totalLines; i++) {
        int lineY = firstLineY + i * lineHeight + dip(2);
        canvas.drawLine(0, lineY, bounds.right, lineY, linePaint);
    }


    canvas.drawLine( (float) dip(64), (float) 0, (float) dip(64), getHeight(), marginPaint );

    super.onDraw(canvas);
    setPadding(dip(64), 0, 0, 0);
}

public void setCursorMoveAllowed(boolean allowed)   {
    moveCursorDisabled = !allowed;
    setCursorVisible(allowed);
    return;
}

public void setBackspaceAllowed(boolean allowed)    {
    deleteCharsDisabled = !allowed;
    return;
}

public boolean superTouchEvent(MotionEvent event)   {
    return super.onTouchEvent(event);
}

public int dip ( int dip )  {
    float d = this.getResources().getDisplayMetrics().density;
    int pixels = (int)(dip * d);        
    return pixels;

}

}