Fix NullReferenceException

[drewnoakes]

FirstOrDefault can return null, so it's not safe to dereference unconditionally.

Observed while debugging VS:

System.NullReferenceException: Object reference not set to an instance of an object.

Microsoft.TemplateEngine.Orchestrator.RunnableProjects.LocalizationModelDeserializer.LoadPostActionModels.AnonymousMethod__4_4((System.Collections.Generic.IEnumerable<string> NameParts, string LocalizedString) s)
System.Linq.Enumerable.WhereSelectEnumerableIterator<(System.Collections.Generic.IEnumerable<string>, string), (System.Collections.Generic.IEnumerable<string>, string)>.MoveNext()
System.Linq.Lookup<string, (System.Collections.Generic.IEnumerable<string>, string)>.Create<(System.Collections.Generic.IEnumerable<string>, string)>(System.Collections.Generic.IEnumerable<(System.Collections.Generic.IEnumerable<string>, string)> source, System.Func<(System.Collections.Generic.IEnumerable<string>, string), string> keySelector, System.Func<(System.Collections.Generic.IEnumerable<string>, string), (System.Collections.Generic.IEnumerable<string>, string)> elementSelector, System.Collections.Generic.IEqualityComparer<string> comparer)
System.Linq.GroupedEnumerable<(System.Collections.Generic.IEnumerable<string>, string), string, (System.Collections.Generic.IEnumerable<string>, string)>.GetEnumerator()
Microsoft.TemplateEngine.Orchestrator.RunnableProjects.LocalizationModelDeserializer.LoadManualInstructionModels(System.Collections.Generic.IEnumerable<(System.Collections.Generic.IEnumerable<string> NameParts, string LocalizedString)> strings)
Microsoft.TemplateEngine.Orchestrator.RunnableProjects.LocalizationModelDeserializer.LoadPostActionModels(System.Collections.Generic.List<(string Key, string Value)> localizedStrings)
Microsoft.TemplateEngine.Orchestrator.RunnableProjects.LocalizationModelDeserializer.Deserialize(Microsoft.TemplateEngine.Abstractions.Mount.IFile file)
Microsoft.TemplateEngine.Orchestrator.RunnableProjects.ScannedTemplateInfo.FindLocalizations()
Microsoft.TemplateEngine.Orchestrator.RunnableProjects.ScannedTemplateInfo.ScannedTemplateInfo(Microsoft.TemplateEngine.Abstractions.IEngineEnvironmentSettings settings, Microsoft.TemplateEngine.Abstractions.IGenerator generator, Microsoft.TemplateEngine.Abstractions.Mount.IFile templateFile)
Microsoft.TemplateEngine.Orchestrator.RunnableProjects.RunnableProjectGenerator.GetTemplatesFromMountPointInternalAsync(Microsoft.TemplateEngine.Abstractions.Mount.IMountPoint source, System.Threading.CancellationToken cancellationToken) Microsoft.TemplateEngine.Orchestrator.RunnableProjects.RunnableProjectGenerator.Microsoft.TemplateEngine.Abstractions.IGenerator.GetTemplatesFromMountPointAsync(Microsoft.TemplateEngine.Abstractions.Mount.IMountPoint source, System.Threading.CancellationToken cancellationToken)
Microsoft.TemplateEngine.Edge.Settings.Scanner.ScanMountPointForTemplatesAsync(Microsoft.TemplateEngine.Edge.Settings.Scanner.MountPointScanSource source, bool logValidationResults, bool returnInvalidTemplates, System.Threading.CancellationToken cancellationToken)
Microsoft.TemplateEngine.Edge.Settings.Scanner.ScanAsync(string mountPointUri, System.Threading.CancellationToken cancellationToken)
Microsoft.TemplateEngine.Edge.Settings.TemplatePackageManager.UpdateTemplateCacheAsync.AnonymousMethod__2(int index)

[joeloff]

Is this something we should consider backporting for 8 and 9?

[drewnoakes]

@joeloff I'm not sure. Seems low risk, but I don't know the impact.

I'm not familiar with the merge policy for this repo. Please merge when makes sense.