我想用sqlite打开我的数据库。但是，当试图从textinputedittext获取数据并将其放入数据库时，它会抛出npe db = this.getWritableDatabase(); 似乎大多数时候这是由 context 为空。但它不是空的，我用调试器检查了它( myDB = new DatabaseHelper(this); )
（关于这个问题，我已经试过了所有其他的问题，我的情况一定有所不同，因为它们没有帮助。）
主活动.java

public class MainActivity extends AppCompatActivity {
    DatabaseHelper myDB;
    TextInputEditText inputEmail;
    TextInputEditText inputPass;
    Button btn_login;
    Button btn_signup;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        try {
            myDB = new DatabaseHelper(this);
        } catch (Exception e) {
            System.out.print("Exception occurred");
        }
        btn_signup = findViewById(R.id.btn_signup);
        btn_login = findViewById(R.id.btn_login);
        addData();
    }

    public void addData() {
        btn_signup.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {

                inputEmail = findViewById(R.id.editTextEmail);
                inputPass = findViewById(R.id.editTextPass);
                //try {
                myDB.insertData(inputEmail.getText().toString(), 
                inputPass.getText().toString());

                Toast.makeText(MainActivity.this, "Data Inserted", 
                Toast.LENGTH_LONG).show();

               // } catch (Exception e) {
               //     e.printStackTrace();
               // }
            }
        });
    }

数据库助手.java

public class DatabaseHelper extends SQLiteOpenHelper {

    public static final String DATABASE_NAME = "Login_android.db";
    public static final String TABLE_NAME = "login_table.db";
    public static final String COL1 = "ID";
    public static final String COL2 = "EMAIL";
    public static final String COL3 = "PASSWORD";

    public DatabaseHelper(Context context) {
        super(context, DATABASE_NAME, null, 1);
        //SQLiteDatabase db = this.getWritableDatabase();
    }

    @Override
    public void onCreate(SQLiteDatabase db) {
        db.execSQL("CREATE TABLE " + TABLE_NAME +" (ID INTEGER PRIMARY KEY 
        AUTOINCREMENT, EMAIL TEXT, PASSWORD TEXT)");
    }

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

    public boolean insertData(String email, String password) {

        SQLiteDatabase db = null;
        try {
            db = this.getWritableDatabase();
        } catch (Exception e) {
            e.printStackTrace();
        }
        //db.isOpen();
        ContentValues contentValues = new ContentValues();
        contentValues.put(COL2, email);
        contentValues.put(COL3, password);
        long result = db.insert(TABLE_NAME, null, contentValues);
        if (result == -1) {
            return false;
        } else {
            return true;
        }
    }
}

例外
12-06 23:39:12.799 26304-26304/com.example.ryan.temporaryname e/androidruntime:致命异常：主进程：com.example.ryan.temporaryname，pid:26304 java.lang.nullpointerexception:尝试调用虚拟方法'long android.database.sqlite.sqlitedatabase.insert（java.lang.string，java.lang.string，com.example.ryan.temporaryname.databasehelper.insertdata（databasehelper）上的空对象引用中。java:45)在com.example.ryan.temporaryname.mainactivity$1.onclick（mainactivity。java:64)在android.view.view.performclick（view。java:6294)在android.view.view$performclick.run（view。java:24770)在android.os.handler.handlecallback（handler。java:790)在android.os.handler.dispatchmessage（handler。java:99)在android.os.looper.loop（looper。java:164)在android.app.activitythread.main（activitythread。java:6494)位于com.android.internal.os.runtimeinit$methodAndArgscaler.run（runtimeinit）的java.lang.reflect.method.invoke（本机方法）。java:438)在com.android.internal.os.zygoteinit.main（zygoteinit。java:807)