[v1.4.2] Fixed error when executing collapsed nodes

The error was caused by using the wrong attribute when retrieving the existing sockets from a collapsed node. Using the correct attribute name fixed the error.

The error was causing the output of collapsed nodes to be handled in the wrong way because it wasn't able to accurately determine the number of existing output sockets. This problem could cause all sorts of other problems, like the graph failing to execute or the wrong data being passed along to other nodes.