title: Why im getting this error Warning: Operand of null-aware operation '??' has type 'Color' which excludes null
body: <p>Im using this package</p>
  <pre><code>flutter_datetime_picker: ^1.5.1
  </code></pre>
  <p>And this is my code</p>
  <pre><code>String _date = &quot;Please pick Age&quot;;
  
    Widget _buildage() {
      return Column(crossAxisAlignment: CrossAxisAlignment.start, children: [
        Text(
          'Enter Age',
          style: kLabelStyle,
        ),
        SizedBox(height: 10.0),
        Container(
          decoration: kBoxDecorationStyle,
          alignment: Alignment.centerLeft,
          height: 70.0,
          child: Container(
            child: TextFormField(
              initialValue: &quot;haaas&quot;,
              validator: (val) {
                if (val.isEmpty) {
                  return 'Enter yout Age';
                }
                if (val.length &lt; 4) {
                  return 'Enter a username minimun 4 chars long';
                }
  
                return null;
              },
              onChanged: (val) {
                setState(() =&gt; age = val);
              },
              onTap: () {
                DatePicker.showDatePicker(context,
                    theme: DatePickerTheme(
                      containerHeight: 210.0,
                    ),
                    showTitleActions: true,
                    minTime: DateTime(2000, 1, 1),
                    maxTime: DateTime(2022, 12, 31), onConfirm: (date) {
                  setState(() {
                    _date = '${date.year} - ${date.month} - ${date.day}';
                  });
                }, currentTime: DateTime.now(), locale: LocaleType.en);
              },
              readOnly: true,
              decoration: InputDecoration(
                border: InputBorder.none,
                contentPadding: EdgeInsets.only(top: 14.0),
                prefixIcon: Icon(
                  Icons.date_range_rounded,
                  color: Colors.white,
                  size: 28,
                ),
                hintText: &quot; $_date&quot;,
                hintStyle: kHintTextStyle,
              ),
            ),
          ),
        ),
      ]);
    }
  </code></pre>
  <p>the error or warning is this</p>
  <pre><code>../../flutter/.pub-cache/hosted/pub.dartlang.org/flutter_datetime_picker-1.5.1/lib/flutter_datetime_picker.dart:311:32: Warning: Operand of null-aware operation '??' has type 'Color' which excludes null.
   - 'Color' is from 'dart:ui'.
                    color: theme.backgroundColor ?? Colors.white,
                                 ^
  
  </code></pre>
  <p>Everything works fine and I dont know why im getting this error  . if you need more information please leave a comment. Hope anyone knows how to fix that . If you need any other information please leave also a comment</p>