Update multiselect.ts
Update multiselect.ts

When I try to import the MultiSelectModule into my application, webpack gives me the following error in production environment with strictNullChecks: "Type 'null' is not assignable to type 'HTMLInputElement'".
carlosearaujo/primeng,primefaces/primeng,sourabh8003/ultimate-primeNg,311devs/primeng,gabriel17carmo/primeng,carlosearaujo/primeng,311devs/primeng,aaraggornn/primeng,primefaces/primeng,sourabh8003/ultimate-primeNg,carlosearaujo/primeng,gilhanan/primeng,gabriel17carmo/primeng,aaraggornn/primeng,sourabh8003/ultimate-primeNg,311devs/primeng,gilhanan/primeng,aaraggornn/primeng,aaraggornn/primeng,carlosearaujo/primeng,WebRota/primeng,gilhanan/primeng,gabriel17carmo/primeng,primefaces/primeng,WebRota/primeng,311devs/primeng,primefaces/primeng,gilhanan/primeng,WebRota/primeng,WebRota/primeng,sourabh8003/ultimate-primeNg,gabriel17carmo/primeng