fix(comms): fix assigment to undefined in QueryGraph.walkDocument

checks if edge.getSource() and edge.getTarget() are undefined when
attempting to set _isSpill on those objects to true

Signed-off-by: Jeremy Clements <79224539+jeclrsg@users.noreply.github.com>

Author: jeclrsg

packages/comms/src/ecl/queryGraph.ts

@@ -753,9 +753,13 @@ export class QueryGraph {
                             } else if (edge._sourceActivity || edge._targetActivity) {
                                 edge._isSpill = true;
                                 const source = edge.getSource();
-                                source._isSpill = true;
+                                if (source) {
+                                    source._isSpill = true;
+                                }
                                 const target = edge.getTarget();
-                                target._isSpill = true;
+                                if (target) {
+                                    target._isSpill = true;
+                                }
                             }
                             retVal.addEdge(edge);
                             break;