fix(core): remove parameter's parent upon remove

fixes #120

Signed-off-by: Ingo Bürk <ingo.buerk@tngtech.com>

Author: Airblader

projects/ngqp/core/src/lib/model/query-param-group.spec.ts

@@ -230,6 +230,7 @@ describe(QueryParamGroup.name, () => {
         it('can remove an existing parameter from a group', () => {
             group.remove('q');
             expect(group.get('q')).toBeNull();
+            expect((stringParam as any).parent).toBe(null);
         });
 
         it('throws if the parameter does not exist', () => {

projects/ngqp/core/src/lib/model/query-param-group.ts

@@ -115,6 +115,7 @@ export class QueryParamGroup {
         }
 
         delete this.queryParams[ queryParamName ];
+        queryParam._setParent(null);
         queryParam._clearChangeFunctions();
     }
 

projects/ngqp/core/src/lib/model/query-param.ts

@@ -9,7 +9,7 @@ abstract class AbstractQueryParamBase<T> {
 
     public abstract value: T;
 
-    protected parent: QueryParamGroup;
+    protected parent: QueryParamGroup | null = null;
     protected _valueChanges = new Subject<T>();
     protected changeFunctions: OnChangeFunction<T>[] = [];
 
@@ -35,7 +35,7 @@ abstract class AbstractQueryParamBase<T> {
     }): void;
 
     public _setParent(parent: QueryParamGroup): void {
-        if (this.parent) {
+        if (this.parent && parent) {
             throw new Error(`Parameter already belongs to a QueryParamGroup.`);
         }