I think I fixed it, the original on my image was missing the assignment (this is approximate, i didn't record the original code exactly):

(dropee isKindOf: TransferMorph) ifTrue: [reference := dropee passenger. externalName := dropee passenger className] ifFalse: [reference := dropee. ].

And instead just left externalName as nil, so when you got to "externalName := externalName isOctetString" it reported an unknown object.

I changed it to: (dropee isKindOf: TransferMorph) ifTrue: [reference := dropee passenger. externalName := dropee passenger className] ifFalse: [reference := dropee. externalName := dropee externalName].

(just adding the assignment to externalName)

This was a Squeak 5.1 32 bit image, You should be able to just look at the code and it's obviously wrong. If you want to reproduce, open a workspace, open any new morph with openInWorld, set the workspace to create textual references to dropped morphs, and drag and drop the morph on the workspace.

I'm trying to build a GUI, pretty much a beginner.

Here's the fixed code: -------------------------------------

acceptDroppingMorph: dropee event: evt inMorph: targetMorph "Return the dropee to its old position, and add a reference to it at the cursor point." | bindingName externalName reference | (dropee isKindOf: TransferMorph) ifTrue: [reference := dropee passenger. externalName := dropee passenger className] ifFalse: [reference := dropee. externalName := dropee externalName]. externalName := externalName isOctetString ifTrue: [externalName] ifFalse: ['a' , externalName]. bindingName := externalName withFirstCharacterDownshifted , reference identityHash printString. targetMorph correctSelectionWithString: bindingName , ' '. (self bindingOf: bindingName) value: reference. (dropee isKindOf: TransferMorph) ifFalse: [dropee rejectDropMorphEvent: evt]. ^ true"success"