Google-sheets – I created a button in a panel (dialogue window), when I push the button I’d like to appear a new panel (dialogue) window

I created a button in Google Spreadsheet. When I push the button a custom window appears. In that window I have two buttons, one to close and one to see a new window. Everything goes well except the second button, to view a new window. I already programmed this. The button with value oefening 1 doesn't work. I want to see a new panel. When I test the function: oefeningshow() it is correct! Can you please help me?

<form>
 <p><label id="naam"> </label> uit <label id="klas"></label> </p>
 <input type="button" value="oefening 1" onclick="oefeningshow()" />     
 <input type="button" value="Close Sidebar" click="google.script.host.close()" />

</form>

 <script> 
 function onOpen() {
   SpreadsheetApp.getUi() // Or DocumentApp or FormApp.
  .createMenu('Dialog')
  .addItem('Open', 'openDialog')
  .addToUi();
  }
   function oefeningshow() {
   var html = HtmlService.createHtmlOutputFromFile('oefening1')
  .setSandboxMode(HtmlService.SandboxMode.IFRAME).setWidth(400)
  .setHeight(300);
  SpreadsheetApp.getUi() // Or DocumentApp or FormApp.
  .showModalDialog(html, 'oefening1');
  }