Fixed an index out of bounds crash when ending a message with a mention

Session/Utilities/MentionUtilities.swift

@@ -115,7 +115,12 @@ public enum MentionUtilities {
                 result.addAttribute(.currentUserMentionBackgroundCornerRadius, value: (8 * sizeDiff), range: mention.range)
                 result.addAttribute(.currentUserMentionBackgroundPadding, value: (3 * sizeDiff), range: mention.range)
                 result.addAttribute(.currentUserMentionBackgroundColor, value: primaryColor.color, range: mention.range)
-                result.addAttribute(.kern, value: (3 * sizeDiff), range: NSRange(location: mention.range.upperBound, length: 1))
+                
+                // Only add the additional kern if the mention isn't at the end of the string (otherwise this
+                // would crash due to an index out of bounds exception)
+                if mention.range.upperBound < result.length {
+                    result.addAttribute(.kern, value: (3 * sizeDiff), range: NSRange(location: mention.range.upperBound, length: 1))
+                }
             }
             
             switch (location, mention.isCurrentUser, theme.interfaceStyle) {