The argument type 'Object' can't be assigned to the parameter type 'ImageProvider

I just updated to Dart2 and Flutter sdk: '>=2.12.0 <3.0.0' and now this if statement breaks:

 decoration: new BoxDecoration(
              shape: BoxShape.circle,
              color: Colors.blueAccent,
              border: Border.all(
                  color: Colors.blueAccent,
                  width: 20.0,
                  style: BorderStyle.solid),
              image: new DecorationImage(
                fit: BoxFit.cover,
                image: myMarkerThumb != 'noImage'
                    ? NetworkImage(myMarkerThumb)
                    : AssetImage('assets/images/noImageAvailable.png'),
              ),
            ),

The argument type 'Object' can't be assigned to the parameter type 'ImageProvider'. ),

I'm just starting with flutter and have no idea where to look else.