Referencing the interface script

[A Former User]

Why can I get a working reference for the interface script in ARP GRAPHICS1 and not in Custom ARP??
Like this...

const var Interface = Synth.getMidiProcessor("Interface");

in the Custom ARP I get:

Script Processor1:! Line 1, column 45: Interface was not found.  {U2NyaXB0IFByb2Nlc3NvcjF8b25Jbml0KCl8NDR8MXw0NQ==}

Thx,
u