GetBuildVersion fails with NullReferenceException when version is missing

[the-ress]

GetBuildVersion fails with NullReferenceException when the version key is missing in version.json

Expected result:
An informative error message.

Actual result:

The "Nerdbank.GitVersioning.Tasks.GetBuildVersion" task failed unexpectedly.
System.NullReferenceException: Object reference not set to an instance of an object.
   at Nerdbank.GitVersioning.VersionOracle..ctor(String projectDirectory, Repository repo, Commit head, ICloudBuild cloudBuild, Nullable`1 overrideBuildNumberOffset, String projectPathRelativeToGitRepoRoot) in c:\Code\Nerdbank.GitVersioning\src\NerdBank.GitVersioning\VersionOracle.cs:line 101
   at Nerdbank.GitVersioning.VersionOracle.Create(String projectDirectory, String gitRepoDirectory, ICloudBuild cloudBuild, Nullable`1 overrideBuildNumberOffset, String projectPathRelativeToGitRepoRoot) in c:\Code\Nerdbank.GitVersioning\src\NerdBank.GitVersioning\VersionOracle.cs:line 40
   at Nerdbank.GitVersioning.Tasks.GetBuildVersion.ExecuteInner()
   at MSBuildExtensionTask.ContextAwareTask.Execute()
   at Microsoft.Build.BackEnd.TaskExecutionHost.Microsoft.Build.BackEnd.ITaskExecutionHost.Execute()
   at Microsoft.Build.BackEnd.TaskBuilder.<ExecuteInstantiatedTask>d__26.MoveNext()

(I forgot to commit the version.json in the subdirectory that specified version.)