chore(bulk-model-sync-lib): check if a node is already deleted to avoid exceptions

INode.remove throws an exception if the node isn't part of the model anymore,
because it was implicitly deleted by deleting one of its ancestors before.
Checking INode.isValid before calling INode.remove() avoids this exception.

Author: slisson

bulk-model-sync-lib/src/commonMain/kotlin/org/modelix/model/sync/bulk/ModelImporter.kt

@@ -94,7 +94,9 @@ class ModelImporter(private val root: INode, private val continueOnError: Boolea
             logger.info { "Removing extra nodes..." }
             nodesToRemove.forEach {
                 doAndPotentiallyContinueOnErrors {
-                    it.remove()
+                    if (it.isValid) { // if it's invalid then it's already removed
+                        it.remove()
+                    }
                 }
             }