make the drop down for "combine with" dynamic

pdb-js/src/app/app.module.ts

@@ -2,6 +2,7 @@ import { BrowserModule } from '@angular/platform-browser';
 import { BrowserAnimationsModule } from '@angular/platform-browser/animations';
 import { NgModule, enableProdMode } from '@angular/core';
 import { HttpClientModule } from '@angular/common/http';
+import { ReactiveFormsModule } from '@angular/forms';
 
 import { AppRoutingModule } from './app-routing.module';
 import { AppComponent } from './app.component';
@@ -26,6 +27,7 @@ import { YAxisRangeComponent } from './y-axis-range/y-axis-range.component';
   imports: [
     BrowserModule,
     AppRoutingModule,
+    ReactiveFormsModule,
     MatSelectModule,MatFormFieldModule, MatInputModule,
     BrowserAnimationsModule,
     HttpClientModule