As per the report published in the website:
In response to queries, SIA said the nose gear of the aircraft retracted while the aircraft was at Gate A17 undergoing a landing gear system check at around 8am on Sunday morning. At the time of the incident, there were no passengers or crew on board, but an engineer was on the aircraft.
Aircraft was undergoing on a landing gear system check, so engineer is using the ladder to reach out the complete system.
Yes, it is ladder. Also you can see some other picture which shows the door tore apart due to landing gear crashing.