Air taxi First Officer reported temporary loss of aircraft control on base leg of a visual approach resulting in a stall warning.

Narrative

I was the second in command (SIC) (pilot flying (PF)) training with my lead Captain (pilot not flying (PNF)) on an empty leg. We were on a repositioning flight from ZZZ1 to ZZZ.Originally we were arriving for Runway XXR; which was then abruptly changed to XYL. The airport was Class D up to X;999 ft. MSL with the customary 200 knot airspeed limitation.ATC kept us pretty high for a while on a right downwind to XYL and initially gave us a descent to X000 ft. MSL. Because of the close proximity to the airport and speed limit restriction; I elected to descend with full airbrakes to manage my descent and speed. I was using the Autopilot on heading mode and vertical speed. Before reaching X000 ft. we were given a clearance for a visual approach to Runway XYL. I was planning on a descent to XX00 ft. MSL (1500 ft. above the airport); before disconnecting the Autopilot and adding slats and flaps; while keeping the speed at or slightly below 200 kts. The field elevation at ZZZ was X;XXY'. My PNF was questioning my altitude choice and told me that 2000-2500 ft. AGL was more appropriate. Aim 4-3-3 dictates 1500 ft. AGL for turbine aircraft. Also; I noticed that the required backup approach was not in the FMS for Runway XYL as per our manual.Then; unexpectedly; while on a right base to XYL; the gear horn started going off immediately followed by the stall buzzer. I looked up and noticed the airplane was leveling off at X000 ft. MSL. I immediately disconnected the Autopilot and added power. The airspeed was near 160 kts. At this point my Captain (PNF) called for the airplane and yelled that the airbrakes were still out. I had no idea and; evidently; neither did he. The aircraft never actually stalled and the recovery was prompt and effective. My Captain (PNF) continued to execute the visual approach to Runway XYL and we landed without incident.In retrospect; I believe the CRM was non-existent and the sterile cockpit below 10;000 ft. was not followed appropriately. My PNF was reprimanding me during the approach as my eyes were mostly outside the cockpit during the visual approach. I did not request that X000 be set in the altitude pre-select after the visual approach clearance nor did my PNF verify any altitudes with me whatsoever.After the flight; my Captain (PNF) told me that he wanted to see how far I was going to let it go. He also told me that I lost my situational awareness; but evidently failed to take any action. If he was not letting the situation deteriorate on purpose; then he was obviously not paying attention and not backing me up as the Captain and Pilot monitoring.We were technically conducting a training flight. The primary responsibility of a Pilot in Command is to ensure the safety of the aircraft; its passengers; crew; and any cargo on board. I do not believe that he satisfied that requirement; which evidently led to our undesirable aircraft state.