Flutter Error: RenderPhysicalModel object was given an infinite size during layout

I'm battling with this issue "RenderPhysicalModel object was given an infinite size during layout.". I have one main file and importing a smaller widget in it. The code of the main file is the following:

  Stack(
            children: [
              Container(
                height: MediaQuery.of(context).size.height - 82.0,
                width: MediaQuery.of(context).size.width,
                color: Colors.transparent, 
              ),
              Positioned(
                top: 75.0, 
                child: 
                  Container(
                    decoration: BoxDecoration(
                      borderRadius: BorderRadius.only(
                        topLeft: Radius.circular(45.0),
                        topRight: Radius.circular(45.0),
                          
                      ),
                      color: Colors.white,
                    ),
                  height: MediaQuery.of(context).size.height - 100.0, 
                  width: MediaQuery.of(context).size.width, 
                ), 
              ), 
              Positioned(
                top: 30.0, 
                left: (MediaQuery.of(context).size.width /2) - 100.0, 
                child: Hero(
                  tag: widget.heroTag, 
                  child: Container (
                    decoration: BoxDecoration(
                      image: DecorationImage(
                        image: AssetImage(widget.heroTag),
                        fit: BoxFit.cover, 
                      )
                    ),
                    height: 200.0, 
                    width: 200.0,
                  ),
                )
              ),
              Positioned(
                top: 250.0,
                left:25.0,
                right:25.0,
                child: Column(
                    mainAxisAlignment: MainAxisAlignment.center,
                    children: <Widget> [
                      Text('Im testing'),
                    ],
                  ),
              ),
              Positioned(
                top: 280,
                left:25.0,
                right:25.0,
                child: Calendar(),
              ),
            ]
          )
        ],

And i am trying to add the widget Calendar () which has the following code:

Container(
        height:70,
        width: 50,
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Text(_dateTime == null ? 'You didnt pick a date' : _dateTime.toString()),
            RaisedButton(
              child: Text('Pick a date'),
              onPressed: () {
                showDatePicker(
                    context: context,
                    initialDate: _dateTime ?? DateTime.now(),
                    firstDate: DateTime(2001),
                    lastDate: DateTime(2021)
                ).then((date) {
                  setState(() {
                    _dateTime = date;
                  });
                });
              },

I don't know why is not rendering the Calendar() widget but it is rendering the Text() widget.

Thank you.