I don't know exactly how your code is structured, but I would advise you to call the function that creates/prepares the labels first and then write your text in those boxes. That way you make sure that the boxes will be ready before you attempt to reference them.
Another way is to use the yield keyword, which waits for a signal before it executes what you want it to execute. More on that here, here and in the docs here.
Your problem could well easily be solved by using the
onready keyword before declaring your labels like this:
onready var lbl_foo = $Label_Foo
That way Godot loads the label first before it does anything to it.
Last but not least, you could create a Timer node and use its timeout() signal to execute your functions after some time has elapsed.
I hope these tips help you. But then again I do not know how your code and nodes are laid out.