Fix error 'IDropdownSettings' only refers to a type, but is being used as a value here.

[Mario-Coxe]

this.dropdownSettings: IDropdownSettings = {
singleSelection: false,
idField: 'item_id',
textField: 'item_text',
selectAllText: 'Select All',
unSelectAllText: 'UnSelect All',
itemsShowLimit: 3,
allowSearchFilter: true
};

for:

this.dropdownSettings = {
singleSelection: false,
idField: 'item_id',
textField: 'item_text',
selectAllText: 'Select All',
unSelectAllText: 'UnSelect All',
itemsShowLimit: 3,
allowSearchFilter: true
};