Using LrHttp in Export dialog

I adding a view in the export dialog with items created from http import:

function WebGUI.galleryView(f, sessions, props)

	local galleryView = {}
	local rowView = {}

	local numOfSessions = 0
	for idx, val in pairs(sessions) do
		if (isEmpty(props.filter)) or (not isEmpty(props.filter) and checkTable(props.filter, val["state"])) then
			table.insert (rowView, 
				f:group_box {
					width = 450,
					f:row { 
						f:checkbox { value = LrView.bind{ bind_to_object = selections, key = "box"..idx }, },
						f:static_text { title = val["state"], },
					},
					f:row {
						f:static_text { title = val["name"], },
					},
				}
			)
			numOfSessions = numOfSessions + 1
		end

		if (0 == numOfSessions % numOfSessionInRow) or (#sessions == idx) then
			table.insert (galleryView, f:row(rowView)) 
			rowView = {}
		end
	end

	return f:column(galleryView)
end

then it is used like this:

function ExportDialogSections.sectionsForBottomOfDialog( f, propertyTable )

	local result
	local bind = LrView.bind
	local webView = nil

	LrFunctionContext.postAsyncTaskWithContext( "showExportDialog", function( context )

			local err = Html.extractGaleries()
			if err then	LrDialogs.message( "HTTP error: ", err )
			else
				webSessions = Html.getSessions()
				WebGUI.initSelectionBinding( webSessions, context )
				webView = WebGUI.galleryView(f, webSessions, propertyTable)
			end
	end )

	local result = {
			{
[...]

				f:column {
					spacing = f:control_spacing(),

					f:scrolled_view {
						width = 550,
						height = 300,
						webView,
					},
				},

So my problem is that i need to run LrHttp inside async task, but I don't know how to make sectionsForBottomOfDialog() wait for the result of async task.