title: Instance member cannot be used on type in SwiftUI Preview
body: <p>Following error in my preview:</p>
  
  <pre><code>struct DetailView: View {
      var header: DataProvider.DataHeader
  
      var body: some View {
          Text("...")
      }
  }
  
  struct DetailView_Previews: PreviewProvider {
      var a = DataProvider.DataHeader(title: "a", text: "b")
  
      static var previews: some View {
          DetailView(header: a)
      }
  }
  </code></pre>
  
  <p>Error is:</p>
  
  <pre><code>Instance member 'a' cannot be used on type 'DetailView_Previews'
  </code></pre>
  
  <p>Why this is happening?</p>