Angular 7 + ngx-bootstrap 3.1.3
First thing is you need to change your import in app.component.ts
from
import { BsModalRef } from 'ngx-bootstrap/modal/bs-modal-ref.service';
to
import { BsModalRef } from 'ngx-bootstrap';
then you will have to provide providers in app.module
import { NgModule } from '@angular/core';
import { BrowserModule } from '@angular/platform-browser';
import { FormsModule } from '@angular/forms';
import { BsModalRef } from 'ngx-bootstrap';
import { AppComponent } from './app.component';
import { HelloComponent } from './hello.component';
import { ModalModule } from 'ngx-bootstrap/modal';
import { BsDropdownModule } from 'ngx-bootstrap/dropdown';
@NgModule({
imports: [ BrowserModule, FormsModule,ModalModule.forRoot(),
BsDropdownModule.forRoot() ],
declarations: [ AppComponent, HelloComponent ],
bootstrap: [ AppComponent ],
providers: [BsModalRef]
})
export class AppModule { }
working STACKBLITZ