NullReferenceException in DefaultTypeConverter

[alexmg]

I am seeing the NullReferenceException below when unit tests are being executed concurrently. It appears some reordering might be causing the double locking that is in place to not behave as expected.

System.NullReferenceException: Object reference not set to an instance of an object.
   at System.Collections.Generic.Dictionary`2.FindEntry(TKey key)
   at System.Collections.Generic.Dictionary`2.TryGetValue(TKey key, TValue& value)
   at Jint.Runtime.Interop.DefaultTypeConverter.TryConvert(Object value, Type type, IFormatProvider formatProvider, Object& converted)
   at Jint.Runtime.Descriptors.Specialized.IndexDescriptor..ctor(Engine engine, Type targetType, String key, Object item)
   at Jint.Runtime.Interop.ObjectWrapper.<>c__DisplayClass9_0.<ResolveProperty>b__3(Engine engine, Object target)
   at Jint.Runtime.Interop.ObjectWrapper.GetOwnProperty(String propertyName)
   at Jint.Native.Object.ObjectInstance.Get(String propertyName)
   at Jint.Engine.GetValue(Reference reference, Boolean returnReferenceToPool)
   at Jint.Runtime.Interpreter.Expressions.JintBinaryExpression.JintGenericBinaryExpression.EvaluateInternal()
   at Jint.Runtime.Interpreter.Expressions.JintExpression.GetValue()
   at Jint.Runtime.Interpreter.Expressions.JintExpression.BuildArguments(JintExpression[] jintExpressions, JsValue[] targetArray)
   at Jint.Runtime.Interpreter.Expressions.JintCallExpression.EvaluateInternal()
   at Jint.Runtime.Interpreter.Expressions.JintExpression.GetValue()
   at Jint.Runtime.Interpreter.Statements.JintExpressionStatement.ExecuteInternal()
   at Jint.Runtime.Interpreter.JintStatementList.Execute()
   at Jint.Engine.Execute(Program program)

I can confirm that the problem is not reproducible when using a ConcurrentDictionary<string, bool> instead of Dictionary<string, bool> with locking. I will submit a PR with these changes for your consideration.