ProjectExplorer: Fix KitManager::deregisterKits

There were some problems regarding removal of the default kit:
  - A precondition of setDefaultKit() was violated.
  - The new default kit was set to a kit that was possibly removed later
    as well (and never updated afterwards).
  - The new default kit was only set if it wasn't null, leaving a
    dangling pointer in m_defaultKit.
Also turned some checks into assertions.

Amends e31a06a0f4.

Done-by: Christian Kandeler <christian.kandeler@qt.io>
Fixes: QTCREATORBUG-30493
Fixes: QTCREATORBUG-30502
Change-Id: I3283f02c90d0ec84b579d7cfdf845b03663f5768
Reviewed-by: Eike Ziller <eike.ziller@qt.io>
Reviewed-by: Alessandro Portale <alessandro.portale@qt.io>
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/plugins/projectexplorer/kitmanager.cpp

@@ -657,20 +657,20 @@ void KitManager::deregisterKits(const QList<Kit *> kitList)
 {
     QTC_ASSERT(KitManager::isLoaded(), return);
     std::vector<std::unique_ptr<Kit>> removed; // to keep them alive until the end of the function
-    Kit *newDefault = nullptr;
+    bool defaultKitRemoved = false;
     for (Kit *k : kitList) {
-        if (!k)
-            continue;
+        QTC_ASSERT(k, continue);
         std::optional<std::unique_ptr<Kit>> taken = Utils::take(d->m_kitList, k);
-        if (!taken)
-            continue;
+        QTC_ASSERT(taken, continue);
+        if (defaultKit() == k)
+            defaultKitRemoved = true;
         removed.push_back(std::move(*taken));
-        if (defaultKit() == k) {
-            newDefault = Utils::findOrDefault(kits(), [](Kit *k) { return k->isValid(); });
-        }
     }
-    if (newDefault)
-        setDefaultKit(newDefault);
+
+    if (defaultKitRemoved) {
+        d->m_defaultKit = Utils::findOrDefault(kits(), &Kit::isValid);
+        emit instance()->defaultkitChanged();
+    }
 
     for (auto it = removed.cbegin(); it != removed.cend(); ++it)
         emit instance()->kitRemoved(it->get());