Getting error while executing getwritabledatabase() in android

Question 1

Error is because you trying to access database with null context. So you need to initialize c1 before passing to Database constructor.

I just Edit your StockTable.java try this

import android.content.ContentValues;
import android.content.Context;

public class StockTable {

    Context c1;
    Database d1;
    final String tablename = "StockTable";
    private String column1 = "Stock_ID";
    private String column2 = "StockName";

    final String stocktable = "CREATE TABLE " + tablename + " (" + column1 + " INTEGER PRIMARY KEY , " + column2 + " TEXT) ";


    public StockTable(Context mContext){
        c1= mContext;
        d1= new Database(c1);
    }
    public ContentValues insert() {


        ContentValues cvi = new ContentValues();
        for (int i = 0; i <= sm.getentry.length(); i++) {
            cvi.put(column1, 1);
            cvi.put(column2, sm.getentry);
        }

        return cvi;
    }

    public void delete() {

    }
}

How to Use?

public class stockmanager extends Activity {

    String getentry = null;
    Database d;
    StockTable st;

    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.stockmanager);

        d = new Database(this.getApplicationContext());
        st = new StockTable(this.getApplicationContext());

        final Button AddStock = (Button) findViewById(R.id.button1);
        final EditText entry = (EditText) findViewById(R.id.editText1);
        final Button BroDetail = (Button) findViewById(R.id.button2);

        AddStock.setOnClickListener(new OnClickListener() {

            @Override
            public void onClick(View v) {
                // TODO Auto-generated method stub
                getentry = entry.getText().toString();
                d.db.insert(st.tablename, null, st.insert());

            }
        });
    }
}

UPDATE YOUR Database Class

public class Database extends SQLiteOpenHelper {

    private static String dbname = "Manager";
    private static int dbversion = 2;
    SQLiteDatabase db;
    private Context mContext;

    public Database(Context context) {

        super(context, dbname, null, dbversion);
        // TODO Auto-generated constructor stub
        mContext = context;
        db = this.getWritableDatabase();
    }

    @Override
    public void onCreate(SQLiteDatabase db) {
        // TODO Auto-generated method stub
        StockTable st = new StockTable(mContext);
        db.execSQL(st.stocktable);
    }

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        // TODO Auto-generated method stub

    }

}

Question 2

You should replace this

String select="Select StockID from Stocktable";

With

String select="Select Stock_ID from Stocktable";

And also initialized your Context C1. Create Constructor that passed current Context into StockTable like:

public StockTable(Context cxnt){
this.c1=cxnt;
d1=new Database(c1);
 }

And add this

 StockTable st=new StockTable(this);

into your stockmanager activity.

Question 3

StockID & Stock_ID are two different column name.

While creating table you are using Stock_ID as column and while firing select query you are using StockID as column, that's your mistake. Your query should be

String select="Select Stock_ID from Stocktable";

And also initialized your Context C1. Create Constructor that passed current Context into StockTable like:

public StockTable(Context cxnt){
this.c1=cxnt;
 }

Question 4

Whenever you are working with the database and helper,

Database database = new Database(this);

You cannot use the class instance as Context until onCreate(). Defer your database and db initialization to onCreate() or later.

Refer these also, to understand it better:

SQlite Database: Unable to find context
nullPointerException for SQLiteOpenHelper.getWritableDatabase(), Android

Question 5

When you instantiate StockTable, these two lines are a problem:

Context c1;
Database d1=new Database(c1);

c1 is not initialized.. there is no context in which to create the database. Pass in a valid context to your constructor, or use a "set" method, then instantiate your database for use. Like this:

public StockTable(Context context) {
 c1 = context;
 d1=new Database(c1);
}

And I would replace:

String select="Select StockID from Stocktable";

with:

String select="Select "+  column1 + " from " + tablename;