In your wizard, something like:

  public WizardPage invokeWizard(WizardHost host) {
    project = host.getBrowser().getActiveUserProject();
    // You might want to check if project is null, meaning no projects
    // are active first; then

    if ( WebContextNode.getNodes( project ).length == 0 ) {
      WizardDialog dialog = new WizardDialog(host.getBrowser(),
          new WebAppWizard() {
            public String getWizardTitle() {
              return super.getWizardTitle() +
                  " (for My JSP Wizard)";
            }
          }
      );
      dialog.show();
      if ( WebContextNode.getNodes( project ).length == 0 ) {
        return null;  // Cancelled WebApp wizard, so do nothing
      }
    }

    // Maybe add pages here, then finally
    return super.invokeWizard(host);
  }