Check for person null in startSpan.

Author: Sam1301

app/src/main/java/com/zulip/android/util/CustomHtmlToSpannedConverter.java

@@ -244,15 +244,18 @@ private static void endFont(SpannableStringBuilder text) {
     }
 
     private static void startSpan(SpannableStringBuilder text, Attributes attributes) {
-        String email;
+        String email = null;
         String stringId = attributes.getValue("data-user-id");
         if (stringId != null) {
             // in case of "@all"
             if (stringId.equals("*")) {
                 email = stringId;
             } else {
                 int id = Integer.parseInt(stringId);
-                email = Person.getById(ZulipApp.get(), id).getEmail();
+                Person person = Person.getById(ZulipApp.get(), id);
+                if (person != null) {
+                    email = person.getEmail();
+                }
             }
         } else {
             // for historical messages, revert to use of this attribute