Hello there,
please forgive me if I've missed an already existing issue for this bug, I found none on my quick search.

Versions + Installed Mods + LOG

KSP version: 1.4.3 (already existed on 1.4.2)
MechJeb version: 2.7.3.0
List of mods:

LOG:
https://www.file-upload.net/download-13115468/output_log.txt.html

Problem:

In the VAB (and SPH), MechJeb displays the wrong SLT. Instead of real SLT, it displays the space TWR:

But on Launchpad / Runway all values are fine again:

Found an exception in the log, don't know if it has sth to do with this:

[MechJeb2] Exception in MechJebModuleStageStats.StartSimulation(): System.NullReferenceException: Object reference not set to an instance of an object
  at MuMech.MechJebModuleStageStats.StartSimulation () [0x00000] in <filename unknown>:0 
  at MuMech.MechJebModuleStageStats.StartSimulation () [0x00000] in <filename unknown>:0

Thanks for your efforts!

Do you see the slider that is set to 70km? Move it back to 0 😁

Ah sh't 😄 🤦‍♂️
Thanks!