diff --git a/src/common/graph/graphelement.ts b/src/common/graph/graphelement.ts
index 85e1d2584bfc26cfbbc9e4b54acbf017d934f9e7..6e0fe1ef4e4d4580915e2deeec12caca3b34ef31 100644
--- a/src/common/graph/graphelement.ts
+++ b/src/common/graph/graphelement.ts
@@ -9,6 +9,8 @@ export class GraphElement<JSONType, HistoryType> extends SerializableItem<
 
     constructor(id = -1, graph: Graph = undefined) {
         super(id);
+        this.equals = this.equals.bind(this);
+
         this.graph = graph;
     }
 
@@ -33,6 +35,10 @@ export class GraphElement<JSONType, HistoryType> extends SerializableItem<
      * @returns True, if given object is identical.
      */
     public equals(other: GraphElement<JSONType, HistoryType>): boolean {
+        if (other == undefined) {
+            return false;
+        }
+
         return other.constructor == this.constructor && other.id == this.id;
     }
 }
diff --git a/src/common/graph/link.ts b/src/common/graph/link.ts
index 100894e30f09f2a4e50090109591992c92b89afa..a69469f09963e573df66a864c461edab5c7e743f 100644
--- a/src/common/graph/link.ts
+++ b/src/common/graph/link.ts
@@ -37,6 +37,9 @@ export class Link
 
     constructor(source?: Node, target?: Node, graph?: Graph) {
         super(0, graph);
+
+        this.equals = this.equals.bind(this);
+
         this.source = source;
         this.target = target;
     }
@@ -94,6 +97,10 @@ export class Link
     }
 
     public equals(other: GraphElement<LinkData, SimLinkData>): boolean {
+        if (other == undefined) {
+            return false;
+        }
+
         if (other.constructor != this.constructor) {
             return false;
         }
diff --git a/src/editor/graph.ts b/src/editor/graph.ts
index 9ea0fd5910c06905edfa1f85234b8a661e7c7606..d55cbd38f58abf67677221365700edde456c77e2 100644
--- a/src/editor/graph.ts
+++ b/src/editor/graph.ts
@@ -15,6 +15,10 @@ export class DynamicGraph extends Common.Graph {
         super(data);
         this.onChangeCallbacks = [];
 
+        super.deleteNode = super.deleteNode.bind(this);
+        super.deleteLink = super.deleteLink.bind(this);
+        super.deleteNodeType = super.deleteNodeType.bind(this);
+
         if (data != undefined) {
             this.history = new History<SimGraphData>(
                 this,
@@ -116,7 +120,7 @@ export class DynamicGraph extends Common.Graph {
     }
 
     public deleteLink(id: number): boolean {
-        return this.delete(id, super.deleteNode);
+        return this.delete(id, super.deleteLink);
     }
 
     getLink(