type 'int' is not a subtype of type 'String' error in Dart

title: new Text(time ?? "Empty"),

should be

title: new Text(time != null ? '$time' : "Empty"),

or

title: new Text('${time ?? "Empty"}'),

The code in following line from your snippet is:

title: new Text(time ?? "Empty"),

While, it should actually look like following:

title: new Text(time?.toString() ?? "Empty"),

As the above answers pointed, the time variable was int while the Text() required an String.

There might be another issue: If the time was null, the null-aware operator ?? will not work as expected. Because the time variable was used before the ?? in time *= 1000 expression.

Therefore, the time *= 1000 should be deleted, and the Text should be like

Text(time == null ? "Empty" : '${time * 1000}')

Note: The time was not modified in this case.