set the error callback to a nice default in case nobody set an error callback and an error happens

actionpack/lib/action_controller/metal/live.rb

@@ -108,7 +108,7 @@ module ActionController
 
     class Buffer < ActionDispatch::Response::Buffer #:nodoc:
       def initialize(response)
-        @error_callback = nil
+        @error_callback = lambda { true }
         super(response, SizedQueue.new(10))
       end
 

actionpack/test/controller/live_stream_test.rb

@@ -313,4 +313,11 @@ module ActionController
       assert_equal 304, @response.status.to_i
     end
   end
+
+  class BufferTest < ActionController::TestCase
+    def test_nil_callback
+      buf = ActionController::Live::Buffer.new nil
+      assert buf.call_on_error
+    end
+  end
 end