When a child scene inherits a parent scene with a C# root node, the
parent scene's export variables appear to assume values set in the
parent scene, in the child scene's Inspector. However, when the child
scene is played, the parent scene's export variables assume default
values.
When a node is created, it inherits its parent C# script's fields from
the map CSharpScriptInstance::script->member_info. However this map was
not initialized outside the editor, and this commit ensured it is. This
fixes issues #36480 and #37581.
This is a manual backport of PR #38638 for 3.2.
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