UML compiler doesn't support functions outside scripts

[ChronoVortex]

Describe the bug

Attempting to compile this GML code in an object's creation event

anon_func = function()
{
    show_debug_message("ANON FUNC");
};

anon_func();

Results in this error

;  EXCEPTION!
;   System.ArgumentException: An item with the same key has already been added. Key: 1
;   at System.Collections.Generic.Dictionary`2.TryInsert(TKey key, TValue value, InsertionBehavior behavior)
;   at System.Collections.Generic.Dictionary`2.Add(TKey key, TValue value)
;   at UndertaleModLib.Decompiler.Disassembler.Disassemble(UndertaleCode code, IList`1 vars, UndertaleCodeLocals locals) in D:\a\UndertaleModTool\UndertaleModTool\UndertaleModLib\Decompiler\Disassembler.cs:line 58
;   at UndertaleModTool.UndertaleCodeEditor.DisassembleCode(UndertaleCode code, Boolean first) in D:\a\UndertaleModTool\UndertaleModTool\UndertaleModTool\Editors\UndertaleCodeEditor.xaml.cs:line 531

Reproducing steps

Attempt to compile a non-script function.

Setup Details

Reproduced on a test project compiled in the latest release of GMS2.
data.zip