Don't crash when analyzing projects without compiler options.

src/Features/Core/Diagnostics/EngineV1/DiagnosticIncrementalAnalyzer.cs

@@ -477,7 +477,31 @@ private bool ShouldRunAnalyzerForClosedFile(CompilationOptions options, bool ope
                 return true;
             }
 
-            return Owner.GetDiagnosticDescriptors(analyzer).Any(d => d.GetEffectiveSeverity(options) != ReportDiagnostic.Hidden);
+            return Owner.GetDiagnosticDescriptors(analyzer).Any(d => GetEffectiveSeverity(d, options) != ReportDiagnostic.Hidden);
+        }
+
+        private static ReportDiagnostic GetEffectiveSeverity(DiagnosticDescriptor descriptor, CompilationOptions options)
+        {
+            return options == null
+                ? MapSeverityToReport(descriptor.DefaultSeverity)
+                : descriptor.GetEffectiveSeverity(options);
+        }
+
+        private static ReportDiagnostic MapSeverityToReport(DiagnosticSeverity severity)
+        {
+            switch (severity)
+            {
+                case DiagnosticSeverity.Hidden:
+                    return ReportDiagnostic.Hidden;
+                case DiagnosticSeverity.Info:
+                    return ReportDiagnostic.Info;
+                case DiagnosticSeverity.Warning:
+                    return ReportDiagnostic.Warn;
+                case DiagnosticSeverity.Error:
+                    return ReportDiagnostic.Error;
+                default:
+                    throw ExceptionUtilities.Unreachable;
+            }
         }
 
         private async Task<bool> ShouldRunAnalyzerForStateTypeAsync(DiagnosticAnalyzerDriver driver, DiagnosticAnalyzer analyzer, StateType stateTypeId, ImmutableHashSet<string> diagnosticIds)