So piecing this together with a couple of your other posts, I believe that you are attempting to have a script that will add the input text from a textbox (edittext) to a listbox. Then when the listbox item is double clicked, if the listbox item's text equals new solid, then you add a new solid to your composition. Is this correct? (Can't add comments yet because I don't have the rep)

If this is correct, the following code may be close to what you're after:

// // ScriptUI Listboxes
var counter = 0;

var window = new Window("palette", "Listbox", undefined);
window.orientation = "column";

var edittext1 = window.add('edittext {properties: {name: "edittext1"}}'); 
    edittext1.text = ""; 
    edittext1.preferredSize.width = 200; 

var listBox = window.add("listbox", undefined, []);
listBox.selection = 0;
listBox.size = [200, 100];

var buttonGroup = window.add("group", undefined, "buttonGroup");
buttonGroup.orientation = "row";
var addButton = buttonGroup.add("button", undefined, "+");
addButton.size = [30, 30];
var minusButton = buttonGroup.add("button", undefined, "-");
minusButton.size = [30, 30];

addButton.onClick = function () {
  counter++;
  if (edittext1.text) { // If not empty
    listBox.add("item", edittext1.text);
  } else {
    listBox.add("item", "Item_" + counter);
  }
}
  

minusButton.onClick = function() {
  if(listBox.selection != null) {
    counter--;
    listBox.remove(listBox.selection);
  }
}

/////////////////////////////////////======================================================================================================
listBox.onDoubleClick = function () {
  // alert (listBox.selection);

  if (listBox.selection = "new solid") {
    alert('newSolid')
    addSolid()
  }
}
    
/////////////////////////////////////======================================================================================================

function addSolid(){ // main function
  myComp = app.project.activeItem;
  mySolid = myComp.layers.addSolid([0,0,0], "Solid", myComp.width, myComp.height,1);
}
    
window.center();
window.show();