assign lambda to a variable and call it later - c++11

Question

It might be a duplicate question, but I couldn't find it, probably due to probably lacking terminology.
Here is the problem: I would like to assign a lambda function to a variable that would be called later on.
class MyClass
{
public:
MyClass( QWdiget* widget );
void doSomething();
private:
QWidget* my_widget;
std::function<QString()> my_text;
}
MyClass::MyClass( QWdiget* widget )
: my_text( [=](){return QString();} )
{
if ( qobject_cast<QLabel*>( widget ) )
{
my_text = [=](){return my_widget ? qobject_cast<QLabel*>( my_widget )->text() : Qstring(); };
}
else if ( qobject_cast<QGroupBox*>( widget ) )
{
my_text = [=](){return my_widget ? qobject_cast<QGroupBox*>( my_widget )->title() : QString(); };
}
else
{
return;
}
my_widget = widget;
}
MyClass::DoSomething()
{
QString str = my_text();
....whatever
}
I am getting a crash and can't find out.
Edit:
Implementing correctly the default value partially solved the issue.
Now, qobject_cast crashes.

Answer 1

Add default implementation like this:
if ( qobject_cast<QLabel*>( widget ) )
{
my_text = [=](){return qobject_cast<QLabel*>( my_wdiget )->text(); };
}
else if ( qobject_cast<QGroupBox*>( widget ) )
{
my_text = [=](){return qobject_cast<QGroupBox*>( my_wdiget )->title(); };
}
else
{
my_text = [=](){ return QString(); };
}