Turn off NullableContextOptions until chapters 1-5 are upgraded to C# 8.0.
MODIFY
Directory.Build.props
Directory.Build.props
<Project>
<PropertyGroup>
- <CSharp8Nullable Condition=" '$(CSharp8Nullable)' == ''">enable</CSharp8Nullable>
+ <CSharp8Nullable Condition=" '$(CSharp8Nullable)' == ''">disable</CSharp8Nullable>
<NullableContextOptions>$(CSharp8Nullable)</NullableContextOptions>
<TreatWarningsAsErrors>$(TreatWarningsAsErrors)</TreatWarningsAsErrors>
</PropertyGroup>

Turn off NullableContextOptions until chapters 1-5 are upgraded to C# 8.0.