Android Edit Text Focus Gone after input things in expandable list view

Question

I have problem with the edit text. I have edit text inside child view for each group, and I can add the new group dynamically, when I key in the edit text for the groups that can fit in the screen initially, the focus is still pointing at the edit text, but when I click edit text to key in for those group that I need to scroll down, after I key in any first character, the edit text focus suddenly gone and point randomly How can this happen? I hope the picture is enough for you to understand. My adapter code:

@Override
    public View getGroupView(final int groupPosition, boolean isExpanded, View convertView, ViewGroup parent) {

        LayoutInflater inflater = (LayoutInflater) c.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
        convertView = inflater.inflate(R.layout.personal_loan_list_group, null);

        TextView titleText = (TextView) convertView.findViewById(R.id.lblListHeader);
        Button deleteBtn = (Button) convertView.findViewById(R.id.deleteBtn);
        titleText.setText("Group" + " " + (groupPosition+1));
        deleteBtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                customer.removePersonalLoan(groupPosition);
                notifyDataSetChanged();
            }
        });
        return convertView;
    }

    @Override
    public View getRealChildView(final int groupPosition, int childPosition, boolean isLastChild, View convertView, ViewGroup parent) {
        LayoutInflater inflater = (LayoutInflater) c.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
        convertView = inflater.inflate(R.layout.personal_loan_list_item, null);

        final CustomEditText installmentText = (CustomEditText) convertView.findViewById(R.id.installmentET);
        installmentText.setTag("amount");

        installmentText.addTextChangedListener(new TextWatcher() {
            @Override
            public void beforeTextChanged(CharSequence s, int start, int count, int after) {
            }

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

            @Override
            public void afterTextChanged(Editable s) {
                try {
                    installmentText.removeTextChangedListener(this);
                    installmentText.setText("999");
                    installmentText.addTextChangedListener(this);
                } catch (Exception ex) {
                    ex.printStackTrace();
                    installmentText.addTextChangedListener(this);
                }
            }
        });

        return convertView;
    }