Cleanup: nested_array should not be accessed directly.

Replace accesses with GetWidget() as documented.

src/airport_gui.cpp

@@ -485,7 +485,7 @@ public:
 				break;
 
 			case WID_AP_AIRPORT_LIST: {
-				int num_clicked = this->vscroll->GetPosition() + (pt.y - this->nested_array[widget]->pos_y) / this->line_height;
+				int num_clicked = this->vscroll->GetPosition() + (pt.y - this->GetWidget<NWidgetBase>(widget)->pos_y) / this->line_height;
 				if (num_clicked >= this->vscroll->GetCount()) break;
 				const AirportSpec *as = AirportClass::Get(_selected_airport_class)->GetSpec(num_clicked);
 				if (as->IsAvailable()) this->SelectOtherAirport(num_clicked);

src/depot_gui.cpp

@@ -1046,7 +1046,7 @@ struct DepotWindow : Window {
 			this->RaiseWidget(WID_D_SELL);
 			this->SetWidgetDirty(WID_D_SELL);
 		}
-		if (this->nested_array[WID_D_SELL] != nullptr && !this->IsWidgetDisabled(WID_D_SELL_CHAIN)) {
+		if (this->GetWidget<NWidgetBase>(WID_D_SELL) != nullptr && !this->IsWidgetDisabled(WID_D_SELL_CHAIN)) {
 			this->RaiseWidget(WID_D_SELL_CHAIN);
 			this->SetWidgetDirty(WID_D_SELL_CHAIN);
 		}

src/object_gui.cpp

@@ -515,7 +515,7 @@ public:
 	{
 		switch (GB(widget, 0, 16)) {
 			case WID_BO_CLASS_LIST: {
-				int num_clicked = this->vscroll->GetPosition() + (pt.y - this->nested_array[widget]->pos_y) / this->line_height;
+				int num_clicked = this->vscroll->GetPosition() + (pt.y - this->GetWidget<NWidgetBase>(widget)->pos_y) / this->line_height;
 				if (num_clicked >= (int)this->object_classes.size()) break;
 
 				this->SelectOtherClass(this->object_classes[num_clicked]);