findViewById returns a null object reference in a listView

Hi everyone I started coding in java since 2 days and now I'm facing a problem, I found topics that were talking about this, but no one has provided me the right solution.

Im trying to create a "checklist using listView and android BasicAdapter, so I got my row layout xml file :

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

    <TextView
        android:id="@+id/procedure_title_text_View"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:textSize="30sp"
        android:textColor="@color/black"
        android:text="@string/procedure_title_default_name"
        android:layout_marginStart="30dp"
        android:layout_marginTop="15dp"/>

    <CheckBox
        android:id="@+id/procedure_title_checkBox"
        android:layout_width="wrap_content"
        android:layout_height="40dp"
        android:layout_alignBaseline="@+id/procedure_title_text_View"
        android:layout_alignParentEnd="true"/>
</RelativeLayout>

When I try to add this row layout to my list view everything works fine but when I try to change the paramaters of this row layout, like the text in the TextView using this code (someone told me to use a ViewHolder but it doesnt changed anything) :

@Override    public View getView(int position, View convertView, ViewGroup parent) {

        ViewHolder holder = null;    
    
        if(convertView == null){
            holder = new ViewHolder();
            LayoutInflater inflater = (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
            convertView = inflater.inflate(R.layout.adapter_procedure, parent, false);

            holder.title = (TextView) convertView.findViewById(R.id.procedure_title_text_View);
            holder.title_checked = (CheckBox) convertView.findViewById(R.id.procedure_title_checkBox);   

            convertView.setTag(holder); 
       }

        else{
            holder = (ViewHolder) convertView.getTag();
        }

        ProcedureTitle currentTitle = getItem(position);
        String titleName = currentTitle.getTitle();
        boolean titleChecked = currentTitle.getChecked();

        holder.title.setText(titleName);
        holder.title_checked.setChecked(titleChecked);

        return convertView;
    }

    private static class ViewHolder {
        TextView title;
        CheckBox title_checked;
    }
}

The application just crashes and gives me an error like this:

    java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.TextView.setText(java.lang.CharSequence)' on a null object reference
        at com.example.protostage2_v1.adapter.ProcedureTitleAdapter.getView(ProcedureTitleAdapter.java:75)

So I tried to run it with the debug mode, and I found that the line (below), returns a null result. The TextView is then not located and causes the crash of the application.

holder.title = (TextView) convertView.findViewById(R.id.procedure_title_text_View);

I'm looking to try to fix this, if you have any idea on how to solve it, I would appreciate your help ! :)