Can't pass a null reference in BeginPopupModal

[NogginBops]

I've found a problem where I want to call BeginPopupModal like this:

ImGui::BeginPopupModal(label, nullptr, flags)

which is possible by doing ref Unsafe.AsRef<bool>(null) in an unsafe context but the marshalling done in this library dereferences that reference which causes a NullRefernceException.

[Critical-Impact]

Would love to see a fix for this as I am running into this now. Can't really see a way around it