Handle null value in assert

src/Compilers/Core/Portable/DiagnosticAnalyzer/AnalyzerManager.AnalyzerExecutionContext.cs

@@ -141,8 +141,7 @@ HostSymbolStartAnalysisScope getSymbolAnalysisScopeCore()
                                 _lazyPendingMemberSymbolsMapOpt = _lazyPendingMemberSymbolsMapOpt ?? new Dictionary<ISymbol, HashSet<ISymbol>>();
 
                                 // Guard against entry added from another thread.
-                                Debug.Assert(!_lazyPendingMemberSymbolsMapOpt.TryGetValue(symbol, out var existingDependentSymbols) ||
-                                    dependentSymbols.SetEquals(existingDependentSymbols));
+                                VerifyNewEntryForPendingMemberSymbolsMap(symbol, dependentSymbols);
                                 _lazyPendingMemberSymbolsMapOpt[symbol] = dependentSymbols;
                             }
                         }
@@ -187,6 +186,23 @@ void processMembers(IEnumerable<ISymbol> members)
                 }
             }
 
+            [Conditional("DEBUG")]
+            void VerifyNewEntryForPendingMemberSymbolsMap(ISymbol symbol, HashSet<ISymbol> dependentSymbols)
+            {
+                if (_lazyPendingMemberSymbolsMapOpt.TryGetValue(symbol, out var existingDependentSymbols))
+                {
+                    if (existingDependentSymbols == null)
+                    {
+                        Debug.Assert(dependentSymbols == null);
+                    }
+                    else
+                    {
+                        Debug.Assert(dependentSymbols != null);
+                        Debug.Assert(dependentSymbols.SetEquals(existingDependentSymbols));
+                    }
+                }
+            }
+
             public void ClearSymbolScopeTask(ISymbol symbol)
             {
                 lock (_gate)