Release activity on destroy callback

Signed-off-by: Simone Arpe <[email protected]>

Author: simoarpe

android/src/main/java/com/pspdfkit/flutter/pspdfkit/FlutterPdfActivity.java

@@ -30,6 +30,12 @@ protected void onCreate(Bundle bundle) {
         bindActivity();
     }
 
+    @Override
+    protected void onDestroy() {
+        super.onDestroy();
+        releaseActivity();
+    }
+
     @Override
     public void onDocumentLoaded(@NonNull PdfDocument pdfDocument) {
         super.onDocumentLoaded(pdfDocument);
@@ -52,6 +58,14 @@ private void bindActivity() {
         currentActivity = this;
     }
 
+    private void releaseActivity() {
+        Result result = loadedDocumentResult.getAndSet(null);
+        if (result != null) {
+            result.success(false);
+        }
+        currentActivity = null;
+    }
+
     public static FlutterPdfActivity getCurrentActivity() {
         return currentActivity;
     }