lvgui: Fix bogus `null?` usage
This `get_children` function was only used in the error applet.
@@ -204,7 +204,7 @@ module LVGL
last_child = nil
loop do
ptr = LVGL.ffi_call!(self.class, :get_child_back, @self_pointer, last_child)
- break if ptr.null?
+ break if ptr.nil?
last_child = LVObject.from_pointer(ptr)
children << last_child
end
