How to use custom image for icons?

[Edward Cui]

Is your feature request related to a problem? Please describe.
Using icon: Image.asset(...) gives the error:

The element type 'BottomNavigationBarItem' can't be assigned to the list type 'SidebarXItem'.dart[list_element_type_not_assignable](https://dart.dev/diagnostics/list_element_type_not_assignable)

Describe the solution you'd like

I would like to be able to use custom image icons.