getDisabledElevation method
The button's elevation when MaterialButton.onPressed is null (when
MaterialButton.enabled is false).
Returns the button's MaterialButton.elevation if it is non-null.
Otheriwse the disabled elevation is 0.0.
Implementation
double getDisabledElevation(MaterialButton button) {
if (button.disabledElevation != null)
return button.disabledElevation;
return 0.0;
}