You should simply wait for the EVENT_ADD_ON_LOADED before doing anything. When that fires, you can be certain that the saved vars have been loaded.
Lua Code:
-- Here MyAddOnSavedVar is nil and if you assign any value it will be overwritten later
EVENT_MANAGER:RegisterForEvent("MyAddOnEventNamespace", EVENT_ADD_ON_LOADED, function(_, addOnName)
if addOnName == "MyAddOn" then
EVENT_MANAGER:UnregisterForEvent("MyAddOnEventNamespace", EVENT_ADD_ON_LOADED)
-- Here MyAddOnSavedVar is either nil if it didn't exist, or the previously stored value
end
end)