Widget Char-Sequence Error in Android Studio

iam a very dumb newbie android programmer, here i have some problem in my case i want to run my android programs that i make on Android Studio IDE. But when i run the program, the error field said "Process: com.example.ever_ncn.cashflow, PID: 6317 java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.TextView.setText(java.lang.CharSequence)' on a null object reference"

I am very confuse cuz of this error, i have googling for it but it still unsolved. Please master, answer my question with simple and easy understanding explanation. Thank you.

NB. My main activity is to show an activity that contain a spinner which is the spinner value is taken from database. This is my source code for Main_Activity.java :

public class MainActivity extends ActionBarActivity {

private static Button BtnINewTrans;
private static Button BtnIViewCash;
private static Button BtnIAddCateg;
DatabaseHelper dbHelper = new DatabaseHelper(this);
Spinner selectCategory;
ArrayAdapter<String> adapterCategory;


@Override
protected void onCreate(Bundle savedInstanceState) {

    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    onButtonClickButtonListener();
    select_spinner_Category();


}

public void select_spinner_Category (){
    ArrayList<String> AllCategoryListing= dbHelper.getAllCategory();
    selectCategory = (Spinner) findViewById(R.id.spnCategSelect);
    adapterCategory = new ArrayAdapter<String>(this, android.R.layout.simple_spinner_item, R.id.spnCategSelect, AllCategoryListing);
    adapterCategory.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
    selectCategory.setAdapter(adapterCategory);
    selectCategory.setOnItemSelectedListener(new AdapterView.OnItemSelectedListener() {
        @Override
        public void onItemSelected(AdapterView<?> parent, View view, int position, long id) {
            Toast.makeText(getBaseContext(), parent.getItemAtPosition(position) + " selected", Toast.LENGTH_LONG).show();
            String currencyValue = String.valueOf(parent.getSelectedItem());
        }

        @Override
        public void onNothingSelected(AdapterView<?> parent) {

        }
    });

}


@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.menu_main, menu);

    return true;
}

public void onButtonClickButtonListener(){
    BtnINewTrans = (Button)findViewById(R.id.btnNewTrans);
    BtnINewTrans.setOnClickListener(
            new View.OnClickListener() {
                @Override
                public void onClick(View v) {
                    Intent intentNewTrans = new Intent ("com.example.ever_ncn.cashflow.NewTransaction");
                    startActivity(intentNewTrans);
                }
            }
    );

    BtnIViewCash = (Button)findViewById(R.id.btnViewCashflow);
    BtnIViewCash.setOnClickListener(
            new View.OnClickListener() {
                @Override
                public void onClick(View v) {
                    Intent intentViewCash = new Intent ("com.example.ever_ncn.cashflow.ViewCashflow");
                    startActivity(intentViewCash);
                }
            }
    );

    BtnIAddCateg = (Button)findViewById(R.id.btnAddCateg);
    BtnIAddCateg.setOnClickListener(
            new View.OnClickListener() {
                @Override
                public void onClick(View v) {
                    Intent intentAddCateg = new Intent ("com.example.ever_ncn.cashflow.AddCategory");
                    startActivity(intentAddCateg);
                }
            }
    );

}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    // Handle action bar item clicks here. The action bar will
    // automatically handle clicks on the Home/Up button, so long
    // as you specify a parent activity in AndroidManifest.xml.
    int id = item.getItemId();

    //noinspection SimplifiableIfStatement
    if (id == R.id.action_settings) {
        return true;
    }

    return super.onOptionsItemSelected(item);
}

}

and this one for database_helper.java :

public class DatabaseHelper extends SQLiteOpenHelper {
public static final String MyVillageSoftware = "MyVillageSoftware";
public static final String DATABASE_NAME = "Cashflow.db";
public static final String TABLE_Categ_NAME = "category_table";
public static final String COL1 = "CategId";
public static final String COL2 = "CategName";
public static final String COL3 = "Note";
public static final String COL4 = "Currency";



public DatabaseHelper(Context context) {
    super(context, DATABASE_NAME, null, 2);


}

@Override
public void onCreate(SQLiteDatabase db) {

    db.execSQL("Create table " + TABLE_Categ_NAME +
            " (CategID Integer PRIMARY KEY AUTOINCREMENT, " +
            "CategName Text," +
            " Note Text," +
            " Currency Text)");

}

public boolean insertCategData(String categname, String note, String currency){
    SQLiteDatabase db = this.getWritableDatabase();
    ContentValues contentValues = new ContentValues();
    contentValues.put(COL2, categname);
    contentValues.put(COL3, note);
    contentValues.put(COL4, currency);
    long result = db.insert(TABLE_Categ_NAME, null, contentValues);
     if (result == -1)
         return true;
     else
         return false;

}
public ArrayList<String>getAllCategory(){
    ArrayList<String> AllCategoryList = new ArrayList<String>();
    SQLiteDatabase db = this.getReadableDatabase();
    String selectCateg="Select * FROM " +TABLE_Categ_NAME;
    Cursor cursor = db.rawQuery(selectCateg, null);
    if(cursor.getCount()>0){
        while (cursor.moveToNext()){
            String categname=cursor.getString(cursor.getColumnIndex(COL2));
            AllCategoryList.add(COL2);

        }return AllCategoryList;
    }

    return AllCategoryList;

}


@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
    db.execSQL("DROP TABLE IF EXISTS " +TABLE_Categ_NAME);
    onCreate(db);

}

}