Crash: NPE in MessageCryptoHelper.addCryptoResultAnnotationToMessage()
Via Play Developer Console
App version: 5.200

```
java.lang.RuntimeException: Failure delivering result ResultInfo{who=null, request=65916, result=0, data=null} to activity {com.fsck.k9/com.fsck.k9.activity.MessageList}: java.lang.NullPointerException: Attempt to read from field 'com.fsck.k9.mail.Part com.fsck.k9.ui.crypto.MessageCryptoHelper$CryptoPart.part' on a null object reference
	at android.app.ActivityThread.deliverResults(ActivityThread.java:3733)
	at android.app.ActivityThread.handleSendResult(ActivityThread.java:3776)
	at android.app.ActivityThread.-wrap16(ActivityThread.java)
	at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1412)
	at android.os.Handler.dispatchMessage(Handler.java:102)
	at android.os.Looper.loop(Looper.java:148)
	at android.app.ActivityThread.main(ActivityThread.java:5461)
	at java.lang.reflect.Method.invoke(Native Method)
	at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
	at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
Caused by: java.lang.NullPointerException: Attempt to read from field 'com.fsck.k9.mail.Part com.fsck.k9.ui.crypto.MessageCryptoHelper$CryptoPart.part' on a null object reference
	at com.fsck.k9.ui.crypto.MessageCryptoHelper.addCryptoResultAnnotationToMessage(MessageCryptoHelper.java:575)
	at com.fsck.k9.ui.crypto.MessageCryptoHelper.onCryptoOperationCanceled(MessageCryptoHelper.java:558)
	at com.fsck.k9.ui.crypto.MessageCryptoHelper.onActivityResult(MessageCryptoHelper.java:536)
	at com.fsck.k9.activity.MessageLoaderHelper.onActivityResult(MessageLoaderHelper.java:168)
	at com.fsck.k9.ui.messageview.MessageViewFragment.onPendingIntentResult(MessageViewFragment.java:393)
	at com.fsck.k9.activity.MessageList.onActivityResult(MessageList.java:1600)
	at android.app.Activity.dispatchActivityResult(Activity.java:6456)
	at android.app.ActivityThread.deliverResults(ActivityThread.java:3729)
	... 9 more
```
diff --git a/k9mail/src/main/java/com/fsck/k9/ui/crypto/MessageCryptoHelper.java b/k9mail/src/main/java/com/fsck/k9/ui/crypto/MessageCryptoHelper.java
index 28387c9f7..2600ddcf0 100644
--- a/k9mail/src/main/java/com/fsck/k9/ui/crypto/MessageCryptoHelper.java
+++ b/k9mail/src/main/java/com/fsck/k9/ui/crypto/MessageCryptoHelper.java
@@ -58,13 +58,14 @@ import org.openintents.openpgp.util.OpenPgpUtils;
 public class MessageCryptoHelper {
     private static final int INVALID_OPENPGP_RESULT_CODE = -1;
     private static final MimeBodyPart NO_REPLACEMENT_PART = null;
-    public static final int REQUEST_CODE_USER_INTERACTION = 124;
-    public static final int PROGRESS_SIZE_THRESHOLD = 4096;
+    private static final int REQUEST_CODE_USER_INTERACTION = 124;
+    private static final int PROGRESS_SIZE_THRESHOLD = 4096;
 
 
     private final Context context;
     private final String openPgpProviderPackage;
     private final Object callbackLock = new Object();
+    private final Deque<CryptoPart> partsToDecryptOrVerify = new ArrayDeque<>();
 
     @Nullable
     private MessageCryptoCallback callback;
@@ -76,7 +77,6 @@ public class MessageCryptoHelper {
 
 
     private MessageCryptoAnnotations messageAnnotations;
-    private Deque<CryptoPart> partsToDecryptOrVerify = new ArrayDeque<>();
     private CryptoPart currentCryptoPart;
     private Intent currentCryptoResult;
     private Intent userInteractionResultIntent;
@@ -554,8 +554,12 @@ public class MessageCryptoHelper {
     }
 
     private void onCryptoOperationCanceled() {
-        CryptoResultAnnotation errorPart = CryptoResultAnnotation.createOpenPgpCanceledAnnotation();
-        addCryptoResultAnnotationToMessage(errorPart);
+        // there are weird states that get us here when we're not actually processing any part. just skip in that case
+        // see https://github.com/k9mail/k-9/issues/1878
+        if (currentCryptoPart != null) {
+            CryptoResultAnnotation errorPart = CryptoResultAnnotation.createOpenPgpCanceledAnnotation();
+            addCryptoResultAnnotationToMessage(errorPart);
+        }
         onCryptoFinished();
     }
 
@@ -579,8 +583,17 @@ public class MessageCryptoHelper {
     }
 
     private void onCryptoFinished() {
-        currentCryptoPart = null;
-        partsToDecryptOrVerify.removeFirst();
+        boolean currentPartIsFirstInQueue = partsToDecryptOrVerify.peekFirst() == currentCryptoPart;
+        if (!currentPartIsFirstInQueue) {
+            throw new IllegalStateException(
+                    "Trying to remove part from queue that is not the currently processed one!");
+        }
+        if (currentCryptoPart != null) {
+            partsToDecryptOrVerify.removeFirst();
+            currentCryptoPart = null;
+        } else {
+            Log.e(K9.LOG_TAG, "Got to onCryptoFinished() with no part in processing!", new Throwable());
+        }
         decryptOrVerifyNextPart();
     }
 
@@ -594,7 +607,7 @@ public class MessageCryptoHelper {
     }
 
     private void cleanupAfterProcessingFinished() {
-        partsToDecryptOrVerify = null;
+        partsToDecryptOrVerify.clear();
         openPgpApi = null;
         if (openPgpServiceConnection != null) {
             openPgpServiceConnection.unbindFromService();
@@ -613,11 +626,13 @@ public class MessageCryptoHelper {
             throw new AssertionError("Callback may only be reattached for the same message!");
         }
         synchronized (callbackLock) {
-            if (queuedResult != null) {
-                Log.d(K9.LOG_TAG, "Returning cached result to reattached callback");
-            }
             this.callback = callback;
-            deliverResult();
+
+            boolean hasCachedResult = queuedResult != null || queuedPendingIntent != null;
+            if (hasCachedResult) {
+                Log.d(K9.LOG_TAG, "Returning cached result or pending intent to reattached callback");
+                deliverResult();
+            }
         }
     }
 
@@ -663,6 +678,8 @@ public class MessageCryptoHelper {
             callback.startPendingIntentForCryptoHelper(
                     queuedPendingIntent.getIntentSender(), REQUEST_CODE_USER_INTERACTION, null, 0, 0, 0);
             queuedPendingIntent = null;
+        } else {
+            throw new IllegalStateException("deliverResult() called with no result!");
         }
     }
 
['k9mail/src/main/java/com/fsck/k9/ui/crypto/MessageCryptoHelper.java']