using Mono.Cecil;

var fooModule = ModuleDefinition.CreateModule("Foo", ModuleKind.NetModule);
var foo = new TypeDefinition("", "Foo", 0, fooModule.TypeSystem.Object);
fooModule.Types.Add(foo);

var mainModule = ModuleDefinition.CreateModule("Main", ModuleKind.NetModule);
mainModule.ModuleReferences.Add(fooModule);
var program = new TypeDefinition("", "Program", TypeAttributes.Class, mainModule.TypeSystem.Object);
program.Fields.Add(new FieldDefinition("foo", FieldAttributes.Static, mainModule.ImportReference(foo)));
mainModule.Types.Add(program);

Unhandled exception. System.NullReferenceException: Object reference not set to an instance of an object.
   at Mono.Cecil.DefaultMetadataImporter.ImportScope(IMetadataScope scope)
   at Mono.Cecil.DefaultMetadataImporter.ImportScope(TypeReference type)
   at Mono.Cecil.DefaultMetadataImporter.ImportType(TypeReference type, ImportGenericContext context)
   at Mono.Cecil.DefaultMetadataImporter.ImportReference(TypeReference type, IGenericParameterProvider context)
   at Mono.Cecil.ModuleDefinition.ImportReference(TypeReference type, IGenericParameterProvider context)
   at Mono.Cecil.ModuleDefinition.ImportReference(TypeReference type)