Creating a VCL form in dll Delphi - delphi

Question

I have this code that is merely trying to create a form in a DLL. I created the DLL and the form through the RAD studio Berlin IDE. I wanted to just put up a blank form to make sure it was working, unfortunately it is crashing with an EAcess violation (or alternately, an EResNotFound exception with message "Resource TSigForm can not be found"), and I can't figure out what is missing.
DLL code:
library SigDLL;
uses
System.SysUtils, System.Classes, Windows, Vcl.Forms, Vcl.Dialogs,
SignatureForm in 'SignatureForm.pas' {SigForm1};
{$R *.res}
var
SigForm: TSigForm;
procedure PrepareSigDLL(AppHandle : HWND); stdcall;
begin
SigForm := TSigForm.Create(nil); // <--------- CRASHES HERE
end;
procedure GetSignature(Variables: PChar); stdcall;
begin
ShowMessage('GetSignature called!');
end;
procedure CloseSigDLL; stdcall;
begin
ShowMessage('CloseSigDLL called!');
end;
exports
PrepareSigDLL,
GetSignature,
CloseSigDLL;
begin
end.
SigForm code:
unit SignatureForm;
interface
uses
Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes, Vcl.Graphics,
Vcl.Controls, Vcl.Forms, Vcl.Dialogs;
type
TSigForm = class(TForm)
private
{ Private declarations }
public
{ Public declarations }
end;
var
SigForm: TSigForm;
implementation
{$R *.dfm}
end.
Generic Host app for DLL:
unit SigDllHost;
interface
uses
Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes, Vcl.Graphics,
Vcl.Controls, Vcl.Forms, Vcl.Dialogs, Vcl.StdCtrls;
procedure PrepareSigDLL(handle: HWND); stdcall; external 'SigDll.dll';
type
TForm2 = class(TForm)
Button1: TButton;
procedure Button1Click(Sender: TObject);
private
{ Private declarations }
public
{ Public declarations }
end;
var
Form2: TForm2;
implementation
{$R *.dfm}
procedure TForm2.Button1Click(Sender: TObject);
begin
PrepareSigDLL(Self.Handle);
end;
end.