title: Jetpack's BiometricPrompt (androidx.biometric.BiometricPrompt) throws NullPointerException
body: <p>I'm using <code>androidx.biometric:biometric:1.0.0-alpha03</code> in a test project and when pushing the <code>negative button</code> I always get the following error message:</p>
  
  <pre><code>java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.CharSequence androidx.biometric.BiometricFragment.getNegativeButtonText()' on a null object reference
      at androidx.biometric.BiometricPrompt$1$1.run(BiometricPrompt.java:329)
      at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1167)
      at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:641)
      at java.lang.Thread.run(Thread.java:764)
  </code></pre>
  
  <p>I think this is a library issue which needs to be fixed as the fragment seems to be null or is there anything I'm doing wrong here?</p>
  
  <pre><code>val prompt = BiometricPrompt(activity, Executors.newSingleThreadExecutor(), object : BiometricPrompt.AuthenticationCallback() {
          override fun onAuthenticationError(errorCode: Int, errString: CharSequence) {}
          override fun onAuthenticationSucceeded(result: BiometricPrompt.AuthenticationResult) {}
          override fun onAuthenticationFailed() {}
  })
  
  val promptInfo = BiometricPrompt.PromptInfo.Builder()
          .setTitle(title)
          .setSubtitle(subtitle)
          .setDescription(description)
          .setNegativeButtonText(activity.getString(android.R.string.cancel))
          .build()
  
  prompt.authenticate(promptInfo)
  </code></pre>