Angular 10 error TS2314: Generic type 'ModuleWithProviders<T>

Some Angular libraries, such as @angular/router and @ngrx/store, implement APIs that return a type called ModuleWithProviders (typically via a method named forRoot()). This type represents an NgModule along with additional providers. Angular version 9 deprecates use of ModuleWithProviders without an explicitly generic type, where the generic type refers to the type of the NgModule.

read the full documentation here (https://next.angular.io/guide/migration-module-with-providers)

open Flatpickr.module.d.ts

and you can put unknown if you don't know the type

  export declare class FlatpickrModule {
   static forRoot(userDefaults?: FlatpickrDefaultsInterface): ModuleWithProviders<unknown>;
   }