Prevent crash

app/src/main/java/eu/faircode/email/MessageHelper.java

@@ -338,6 +338,7 @@ public class MessageHelper {
 
                         // Build message
                         ContentType ct = new ContentType("multipart/signed");
+                        if (micalg != null)
                             ct.setParameter("micalg", micalg);
                         ct.setParameter("protocol", "application/pgp-signature");
                         String ctx = ct.toString();