How to reproduce:
1. Create a new empty Unity project
2. Add the addressable package to the manifest: "com.unity.addressables": "0.4.8-preview".
3. Go back to Unity to import the package.
4. Open Window -> Asset Management -> Addressable Assets.
5. Click on "Create Addressables Settings" to initialize the configuration.
6. In the Addressables Window, create a new group by right-clicking anywhere -> Create new group -> Packed Assets
7. Close Unity and delete the Library folder
8. Reopen the project and observe the Console Window

Expected result: No errors are thrown after reopening the project
Actual result: Inspector Window functionality gets broken as well as NullReferenceException errors are thrown when clicking on any assets in the Project Window

Reproduced with: 0.4.8-preview; 0.4.6-preview
Not reproducible with: 0.3.5-preview;

The user has also suggested this workaround:

When a new AA group is created, one or more Schemas files associated to this group are also created under ./Assets/AddressableAssetsData/AssetGroups/Schemas.
If you open a problematic .asset file in this folder, you'll notice the following line:
> m_group: {fileID: 0}
From what I understand, this value is supposed to contain the guid of the group related to this schema. If I edit it by hand to set the proper group ID as follow, then everything works fine afterward.
> m_group: {fileID: 11400000, guid: 682dc9cec1fe35d4fa66ad26ea7d468f, type: 2}
The groupID is the guid in the related ./Assets/AddressableAssetsData/AssetGroups/*.asset.meta file.