fix(custom-local-timelines): crash whenever a mention is shown

I don't know what caused this. Probably the HTML parser, but its now fixed forever, hopefully this thing does not happen again :)

mastodon/src/main/java/org/joinmastodon/android/model/Mention.java

@@ -34,7 +34,10 @@ public class Mention extends BaseModel{
 
 	@Override
 	public int hashCode(){
-		int result=id.hashCode();
+		int result = 0;
+		if (id != null) {
+			result=id.hashCode();
+		}
 		result=31*result+url.hashCode();
 		return result;
 	}