Fix crash for missing BuildSystem

There is no gurantee that a Target has a BuildSystem.
Target::additionalData() does expect a BuildSystem.

Task-number: QTCREATORBUG-24817
Change-Id: I41edf89fa6dbf6ed24a27129b8353a9506b7b176
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/plugins/projectexplorer/target.cpp

@@ -775,7 +775,10 @@ void Target::setNamedSettings(const QString &name, const QVariant &value)
 
 QVariant Target::additionalData(Utils::Id id) const
 {
-    return buildSystem()->additionalData(id);
+    if (const BuildSystem *bs = buildSystem())
+        return bs->additionalData(id);
+
+    return {};
 }
 
 MakeInstallCommand Target::makeInstallCommand(const QString &installRoot) const