Object can't be converted from Json array

This is demo generated TRootClass:
```pascal
TRootClass = class
private
  FItems: TArray<TItemClass>;
public
  property Items: TArray<TItemClass> read FItems write FItems;
  destructor Destroy; override;
  function ToJsonString: string;
  class function FromJsonString(AJsonString: string): TRootClass;
end;
```
When calling TRootClass.FromJsonString, it will throw exception.