TypeError: Cannot read property 'key' of undefined

[knowuh]

View details in Rollbar: https://rollbar.com/concordconsortium/Sage/items/1253/

TypeError: Cannot read property 'key' of undefined
  File "webpack:///./src/code/models/link.ts", line 70, in t.terminalKey
        return `${this.sourceNode.key} ------> ${this.targetNode.key}`;
  File "webpack:///./src/code/stores/graph-store.ts", line 203, in c.hasLink
        return (this.linkKeys[link.terminalKey()] != null);
  File "webpack:///./src/code/stores/graph-store.ts", line 244, in c._addLink
        if ((link.sourceNode !== link.targetNode) && !this.hasLink(link)) {
  File "webpack:///./src/code/stores/graph-store.ts", line 237, in Object.execute
          execute: () => this._addLink(link),
  File "webpack:///./src/code/utils/undo-redo.ts", line 236, in e._call
          return this.methods[method]();
  File "webpack:///./src/code/utils/undo-redo.ts", line 242, in e.execute
      public execute(debug) { return this._call("execute", debug); }
  File "webpack:///./src/code/utils/undo-redo.ts", line 92, in e.execute
        const result = command.execute(this.debug);
  File "webpack:///./src/code/utils/undo-redo.ts", line 79, in e.createAndExecuteCommand
        const result = this.execute((new UndoRedoCommand(name, methods)));
  File "webpack:///./src/code/stores/graph-store.ts", line 236, in c.addLink
        return this.undoRedoManager.createAndExecuteCommand("addLink", {
  File "webpack:///./src/code/stores/graph-store.ts", line 230, in c.importLink
        this.addLink(link);