Fix Event<T> argument

[terurou]

This PR fixes a compile error in the following code:

import vscode.ExtensionContext;
import vscode.ViewColumn;

function activate(context:ExtensionContext) {
	final panel = Vscode.window.createWebviewPanel("viewtype", "title", ViewColumn.Active);
	panel.onDidDispose(
            _ -> {},
            null,
            context.subscriptions // <- type error
	);
}