### Godot version

v4.2.rc.custom_build [d6a1db2b0]

### System information

Godot v4.2.rc (d6a1db2b0) - Windows 10.0.22621 - Vulkan (Forward+) - dedicated NVIDIA GeForce RTX 3090 Ti (NVIDIA; 31.0.15.4592) - AMD Ryzen 9 7950X 16-Core Processor (32 Threads)

### Issue description

`AnimatedValuesBackup::get_data()` can crash as the internal data map can have nullptr values if any of the tracks were METHOD or ANIMATION types, as `AnimatedValuesBackup::get_cache_copy()` defines that these types of tracks do not get valid copies.

The resulting crash is therefore a null pointer exception.

### Steps to reproduce

1. Open a project with these type of animation tracks.
2. Save the project.

### Minimal reproduction project

https://github.com/godotengine/godot-demo-projects/tree/master/3d/platformer
callback_mode_method = p_mode;
#ifdef TOOLS_ENABLED
	emit_signal(SNAME("mixer_updated"));
#endif 
}
AnimationMixer::AnimationCallbackModeMethod AnimationMixer::get_callback_mode_method() const {
	return callback_mode_method;
}
void AnimationMixer::set_audio_max_polyphony(int p_audio_max_polyphony) {
	ERR_FAIL_COND(p_audio_max_polyphony < 0 || p_audio_max_polyphony > 128);
	audio_max_polyphony = p_audio_max_polyphony;
}
int AnimationMixer::get_audio_max_polyphony() const {
	return audio_max_polyphony;
}
#ifdef TOOLS_ENABLED
void AnimationMixer::set_editing(bool p_editing) {
	if (editing == p_editing) {
		return;
	}
	editing = p_editing;
	_set_process(processing, true);
	if (editing && is_inside_tree()) {
		_clear_caches();
	}
	notify_property_list_changed(); 
}
bool AnimationMixer::is_editing() const {