Why am I getting a crash when I let the user leaving the field blank and entering a negative values at the same time as leaving some field blank

public void add() {

    // declaring and initializing the blank for if statement
    String blank = "";

    // getting the informations from the imputs
    firsts = first.getText();
    lasts = last.getText();
    s1 = w1.getText();
    s2 = w2.getText();
    s3 = w3.getText();
    s4 = w4.getText();

    // equalling the same variables so it will help to make if statement work properly

    fs = firsts;
    ls = lasts;
    ss1 = s1;
    ss2 = s2;
    ss3 = s3;
    ss4 = s4;

    iw1 = Integer.parseInt(s1); // converting from string to integer for week 1
        iw2 = Integer.parseInt(s2); // converting from string to integer for week 2
        iw3 = Integer.parseInt(s3); // converting from string to integer for week 3
        iw4 = Integer.parseInt(s4); // coverting from string to integer for week 4

    if (fs.equals(blank) || ls.equals(blank) || ss1.equals(blank) || ss2.equals(blank) || ss3.equals(blank) || ss4.equals(blank)) {
        JOptionPane.showMessageDialog(null, "You have not type in one or more blank fields. Please try again.");// display the mesage
    } else if (iw1 < 0 || iw2 < 0 || iw3 < 0 || iw4 < 0) {

        JOptionPane.showMessageDialog(null, "You typed the point(s) in negative value(s). Please try again by entering a positive value(s).");// display the mesage
    } // adding the new passenger info into the data
    else {


        firstn.add(firsts);
        lastn.add(lasts);
        we1.add(iw1);
        we2.add(iw2);
        we3.add(iw3);
        we4.add(iw4);
    }

}

I am getting a crash on the program when I hit add button when I left some field blanks and putting - values for week 1, 2, 3, and 4. Can you help me please. aaaaaaaaaysdioyudewdwdewywdwey8wed9wyde9wdy8e89yw9dewy8d9dweyd8e9wdy8e8dy8e98wdy89e8yd8y9ey8dy8ey8yd8y8e8d8yywydy8y8dy8yedeyd88yed8ye8d8eyd8de8edwy8ey0edwyewwy80w8yey8dy8edydyedshuchiudhucudchdhcudgiyf989fy8e8dyd9ydfchudhchudhuucduhhudhhdudud98e8de8wy8dywyy8eyy8eyyeyfe9y77fhcdc8jc8jcj8c8ew0e0we88fufh