fix(gui): avoid null-pointer UB in create_scaled_bitmap with win == nullptr
create_scaled_bitmap() documents that win may be nullptr, but called
win->FromDIP() on it. Calling a member function through a null pointer
is undefined behavior: clang assumes `this` is non-null and deletes the
subsequent `win ?` null check added in #13117, turning the fallback
branch into an unconditional virtual call through a null vtable.
This crashed LLVM/clang-cl builds at startup (access violation reading
0x0 in BBLTopbar creation); MSVC builds were unaffected by luck.
Use the static, null-safe wxWindow::FromDIP(x, win) overload instead,
which falls back to the primary display DPI. Behavior is unchanged for
non-null windows.
Gabriel Monteiro