Help app crash after button click

[Sunil Bhagwat]

In my application below are codes my app crash on button click

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

Button calculate = (Button) findViewById(R.id.button1);

calculate.setOnClickListener(new OnClickListener() {

public void onClick(View view) {

EditText ht = (EditText) findViewById(R.id.height);
EditText wt = (EditText) findViewById(R.id.weight);
TextView tv7 = (TextView) findViewById(R.id.textView7);
Double x = Double.parseDouble(ht.getText().toString());

Double y = Double.parseDouble(wt.getText().toString());

String z = Double.toString(Math.round( y / (x / 100) * (x / 100)));

if (((ht.getText().toString().isEmpty()) && (wt.getText().toString().isEmpty())) || (wt.getText().toString().isEmpty()) || (ht.getText().toString().isEmpty())) {

tv7.setText("Please enter valid value");
}
}
pl help.