Updated so warnings appear as warnings by default.  The build treats, however, will continue to treat warnings as errors.
MODIFY
Directory.Build.props
Directory.Build.props
<PropertyGroup>
<Nullable>enable</Nullable>
<ProductName Condition="'$(ProductName)'==''">Chapter$(ChapterNumber)</ProductName>
- <TreatWarningsAsErrors Condition="'$(TreatWarningsAsErrors)'==''">true</TreatWarningsAsErrors>
+ <TreatWarningsAsErrors Condition="'$(TreatWarningsAsErrors)'==''">false</TreatWarningsAsErrors>
<TargetFramework>netcoreapp3.0</TargetFramework>
<Authors>Mark Michaelis</Authors>
<Company>Addison Wesley</Company>

Updated so warnings appear as warnings by default.  The build treats, however, will continue to treat warnings as errors.