enhance ParticleCoreShellItem to allow un-setting core & shell

GUI/Models/ParticleCoreShellItem.cpp

@@ -66,9 +66,15 @@ ParticleItem* ParticleCoreShellItem::core() const
     return dynamic_cast<ParticleItem*>(getItem(T_CORE));
 }
 
-void ParticleCoreShellItem::setCore(ParticleItem* core)
+void ParticleCoreShellItem::setCore(ParticleItem* newCore)
 {
-    model()->moveItem(core, this, -1, T_CORE);
+    if (newCore == nullptr) {
+        if (core() != nullptr)
+            model()->removeItem(core());
+        return;
+    }
+
+    model()->moveItem(newCore, this, -1, T_CORE);
 }
 
 ParticleItem* ParticleCoreShellItem::createCore()
@@ -86,9 +92,15 @@ ParticleItem* ParticleCoreShellItem::shell() const
     return dynamic_cast<ParticleItem*>(getItem(T_SHELL));
 }
 
-void ParticleCoreShellItem::setShell(ParticleItem* shell)
+void ParticleCoreShellItem::setShell(ParticleItem* newShell)
 {
-    model()->moveItem(shell, this, -1, T_SHELL);
+    if (newShell == nullptr) {
+        if (shell() != nullptr)
+            model()->removeItem(shell());
+        return;
+    }
+
+    model()->moveItem(newShell, this, -1, T_SHELL);
 }
 
 ParticleItem* ParticleCoreShellItem::createShell()