fix undef check, doc

Author: vankeisb

doc/quickstart.md

@@ -12,19 +12,19 @@ You'll need to setup the project before you get to actual coding.
 You need a React project in order to start using tea-cup.
 In order to keep things simple, we'll use the infamous `create-react-app` :
 
-    npx create-react-app my-app --typescript
+    yarn create react-app my-app --template typescript
     cd my-app
 
 ### Add tea-cup to your dependencies
 
-    npm install react-tea-cup --save
+    yarn add -D react-tea-cup tea-cup-core
 
 ### Run the dev server
 
 We'll start the dev server, and let it recompile everything 
 when we save files :
 
-    npm start
+    yarn start
 
 This should also open your default browser to http://localhost:3000
 and show you the default generated app.

tea-cup/src/TeaCup/Program.ts

@@ -153,7 +153,7 @@ export class Program<Model, Msg> extends Component<ProgramProps<Model, Msg>, nev
   }
 
   render(): ReactNode {
-    if (this.currentModel && this.bd) {
+    if (this.currentModel !== undefined && this.bd) {
       return this.props.view(this.bd, this.currentModel);
     }
     return null;