title: Flutter - The Method '*' was called on Null
body: <p>Working on a crypto project, I am getting errors in my widget, the cause of the error is the <code>getBalance method</code> below</p>
  <pre><code> String getBalance() {
      String _unit = getUnit(); // it returns a string 
      return boolFrom
          ? (balances[_unit]).toStringAsFixed(4)
          : ((balances[_unit] * rates[_unit]) ?? 0).toStringAsFixed(2);
    }
  </code></pre>
  <p>When I tried calling getBalance onInit State</p>
  <pre><code> @override
    void initState() {
      super.initState();
      print('Get Balance is ${getBalance()}');
    }
  </code></pre>
  <p>I get the error <code>NoSuchMethodError: The method '*' was called on null</code></p>