Check activity isFinishing before authentication (#807)

packages/local_auth/android/src/main/java/io/flutter/plugins/localauth/LocalAuthPlugin.java

diff --git a/packages/local_auth/android/src/main/java/io/flutter/plugins/localauth/LocalAuthPlugin.java b/packages/local_auth/android/src/main/java/io/flutter/plugins/localauth/LocalAuthPlugin.java
index f56806c..12e05af 100644
--- a/packages/local_auth/android/src/main/java/io/flutter/plugins/localauth/LocalAuthPlugin.java
+++ b/packages/local_auth/android/src/main/java/io/flutter/plugins/localauth/LocalAuthPlugin.java
@@ -41,7 +41,7 @@
         return;
       }
       Activity activity = registrar.activity();
-      if (activity == null) {
+      if (activity == null || activity.isFinishing()) {
         result.error("no_activity", "local_auth plugin requires a foreground activity", null);
         return;
       }