Fix NPE on BiometricPrompt instance (#141)

* update lock

* check fragment activity

Author: iagormoraes

android/src/main/java/com/hieuvp/fingerprint/ReactNativeFingerprintScannerModule.java

@@ -94,7 +94,7 @@ public void onAuthenticationSucceeded(@NonNull BiometricPrompt.AuthenticationRes
         }
     }
 
-    public BiometricPrompt getBiometricPrompt(final Promise promise) {
+    public BiometricPrompt getBiometricPrompt(final FragmentActivity fragmentActivity, final Promise promise) {
         // memoize so can be accessed to cancel
         if (biometricPrompt != null) {
             return biometricPrompt;
@@ -104,7 +104,6 @@ public BiometricPrompt getBiometricPrompt(final Promise promise) {
         mReactContext.addLifecycleEventListener(this);
 
         AuthCallback authCallback = new AuthCallback(promise);
-        FragmentActivity fragmentActivity = (FragmentActivity) getCurrentActivity();
         Executor executor = Executors.newSingleThreadExecutor();
         biometricPrompt = new BiometricPrompt(
             fragmentActivity,
@@ -120,7 +119,11 @@ private void biometricAuthenticate(final String title, final String subtitle, fi
             new Runnable() {
                 @Override
                 public void run() {
-                    BiometricPrompt bioPrompt = getBiometricPrompt(promise);
+                    FragmentActivity fragmentActivity = (FragmentActivity) mReactContext.getCurrentActivity();
+
+                    if(fragmentActivity == null) return;
+
+                    BiometricPrompt bioPrompt = getBiometricPrompt(fragmentActivity, promise);
 
                     PromptInfo promptInfo = new BiometricPrompt.PromptInfo.Builder()
                         .setDeviceCredentialAllowed(false)