godot: Godot crash in ProceduralSky::_update_sky() after opening a project

Godot version: 3.2 3f76d2c

OS/device including version: Ubuntu 18.04

Issue description: When I try to open project, then this backtrace is shown:

[1] /lib/x86_64-linux-gnu/libc.so.6(+0x3ef20) [0x7fdd76f92f20] (??:0)
[2] ProceduralSky::_update_sky() (/home/rafal/Pulpit/godot/scene/resources/sky.cpp:420)
[3] MethodBind0::call(Object*, Variant const**, int, Variant::CallError&) (/home/rafal/Pulpit/godot/./core/method_bind.gen.inc:56 (discriminator 4))
[4] Object::call(StringName const&, Variant const**, int, Variant::CallError&) (/home/rafal/Pulpit/godot/core/object.cpp:942 (discriminator 1))
[5] MessageQueue::_call_function(Object*, StringName const&, Variant const*, int, bool) (/home/rafal/Pulpit/godot/core/message_queue.cpp:256)
[6] MessageQueue::flush() (/home/rafal/Pulpit/godot/core/message_queue.cpp:305)
[7] SceneTree::iteration(float) (/home/rafal/Pulpit/godot/scene/main/scene_tree.cpp:477 (discriminator 2))
[8] Main::iteration() (/home/rafal/Pulpit/godot/main/main.cpp:1862)
[9] OS_X11::run() (/home/rafal/Pulpit/godot/platform/x11/os_x11.cpp:3006)
[10] /usr/bin/godot(main+0xdc) [0x1323643] (/home/rafal/Pulpit/godot/platform/x11/godot_x11.cpp:56)
[11] /lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xe7) [0x7fdd76f75b97] (??:0)
[12] /usr/bin/godot(_start+0x2a) [0x13234aa] (??:?)

Minimal reproduction project: https://github.com/w84death/mystic-treasure-hunt