Firmware upgrade for GS752TPSB results in (null)
Just purchased two GS752TPSB devices and they came shipped on 220.127.116.11 so I downloaded the 18.104.22.168 firmware. I "downloaded" the stk file to the switch as image2 and then set image2 to be the next version. I decided to just reboot the 2nd switch in the stack and upon reboot the page is reporting (null) for both Boot Version and Software Version.
Re: Firmware upgrade for GS752TPSB results in (null)
I don't have your switch models, but I think by rebooting just the 2nd switch (if I read your post correctly) this the cause for your problem: Even on the bigger M4300 series the firmware needs to be the identical on all stack members in order to join the stack. Otherwise there will be a firmware code mismatch. You'll have to reboot all stack members to the new version in order to get them back into the stack.
In best case checking the dual firmware (activate image) on unit 1 and then rebooting it is all you need to do. If the stack doesn't re-form, you might have to reboot the complete stack by unplugging both and do a cold start into the new firmware.
Similar to my M4300's the release notes for your model and firmware version show that after verifying the image has been sent properly to all stack members and the right firmware bank is enabled, that you should reboot all stack members: https://kb.netgear.com/000038648/GS728TS-GS752TS-GS728TPS-and-GS752TPS-Firmware-Version-5-3-0-31
People at Netgear have outlined a staged update procedure for the M4300 series in this forum but it comes with some difficulties and disadvantages (I do it but only after testing it during a maintenance window). However there is a reason as to why the release notes on all Netgear stackable switches that I've seen so far, recommend rebooting the complete stack at once. It's the most reliable way to properly initialize the stack with the new firmware.