Cannot 'find' on mutableListOf - kotlin

Question

The mytable object contains the columns variable (the CollectionColumns class) and the list variable is declared in this class, which contains objects of the Column class.
I try to search in this list by the variable name and don't find it (returns null).
val myTable = Table()
myTable.columns.add("Date")
myTable.columns.add("Name")
myTable.columns.add("Summ")
val find = myTable.columns.find("Summ")
println(find)
result: null
Does anyone know why ?
class Table() {
var columns: CollectionColumns
var rows: CollectionRows
init {
columns = CollectionColumns()
rows = CollectionRows()
}
inner class CollectionColumns() {
var list = mutableListOf<Column>()
fun add(_name: String): Table.Column{
val newColumn = Column(_name)
columns.list.add(newColumn)
for (row in rows.list) {
row.data.put(newColumn.name, null)
}
return newColumn
}
fun find(_name: String): Column?{
val findColumn = columns.list.find { it.name == _name }
return findColumn
}
}
class Column(_name: String) {
val name = _name
}
inner class CollectionRows() {
var list = mutableListOf<Row>()
fun add() : Table.Row {
val newRow = Row()
for (column in columns.list) {
newRow.data.put(column.name, null)
}
rows.list.add(newRow)
return newRow
}
}
class Row() {
var data = mutableMapOf<String, Any?>()
}
}