Question: I have a widget which has `String Function() validation` as one of its properties.

The widget looks something like

```
MyWidget(validation: () {
      if (text.isEmpty) {
        return "Text is required";
      }  else {
        return null;
      }
    },
  );

```

The problem is when i try to access MyWidget.validation i am expecting it to give me the string but i get an error which says
`A value of type 'String Function()' can't be assigned to a variable of type 'String'.`

Can someone please show me a way out? Thanks

Answer: 
think i found the solution.
i don't know is it ok to do this, but it worked for me.

so, you have two explorer.exe in your windows
one in `C:\Windows\System32` and the other in `C:\Windows`.

when you want to open the first one it shows

> 
> This Application only runs under WinPE or WinRE.
> 
> 
> 

error.
but second one is ok and work properly.

so i just copy `C:\Windows\explorer.exe` to `C:\Windows\System32` and replace it.

> 
> **Note** that it's better to keep the original file somewhere in case it cause a problem.
> 
> 
> 

and now it works fine.

hope i helped.
As far as I'm told it's the Common Item Dialog, not explorer that opens up your browser. So the solution posted by ahmadreza moodi did not help me while solving this issue.

This error appears in most programs that you try to browse files not only Android Studio, but it will also happen even if you try to open a folder when using cmd.

What helped me is only a Windows Repair Install, but it only lasted a few days before the error was back. I currently have no idea how to solve it permanently.