improve invalid child name error

Author: mat-hek

lib/membrane/children_spec.ex

@@ -324,8 +324,8 @@ defmodule Membrane.ChildrenSpec do
     do_child(child_name, child_definition, options)
   end
 
-  def child(_first_arg, _second_arg, _third_arg) do
-    raise "Improper child name! Perhaps you meant to use get_child/2 while building your link?"
+  def child(child_name, _second_arg, _third_arg) do
+    raise "Invalid child name #{inspect(child_name)}. Child name can be an atom or a tuple."
   end
 
   @doc """

lib/membrane/core/child/pad_model.ex

@@ -12,7 +12,7 @@ defmodule Membrane.Core.Child.PadModel do
           ref: Membrane.Pad.ref_t(),
           options: Membrane.ChildrenSpec.pad_options_t(),
           link_id: Membrane.Core.Parent.Link.id(),
-          endpoint: Membrane.Core.Parent.Link.Endpoint.t(),
+          endpoint: Membrane.Core.Parent.Link.Endpoint.t() | nil,
           linked?: boolean(),
           response_received?: boolean(),
           spec_ref: Membrane.Core.Parent.ChildLifeController.spec_ref_t(),