You can’t mix the different types of packages. I think the “@” packages are for ES6. So, you should remove “babylonjs” and all the “babylonjs-xxx” packages and add the corresponding ES6 packages ("@babylonjs/materials" and “@babylonjs/serializers” - Babylon.js ES6 support with Tree Shaking | Babylon.js Documentation).
As @Evgeni_Popov said, it seems like you are including babylon twice - as es66 dependency and as UMD (@babylonjs/core and babylonjs).
You might be using the UMD version to render your scene, but the es6 to display the inspecor. How exactly - it’s hard to say, but try being consistent with your includes. materials and serializers should also come from the @babylonjs namespace instead of the UMD versions.