ProjectExplorer: Disable copying steps to inactive kits

This fixes a hard crash due to a Target nullptr.

Change-Id: I3065ac61bc950413d8958bcad5420f5868afd35c
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>

src/plugins/projectexplorer/project.cpp

@@ -276,6 +276,7 @@ Target *Project::createTarget(Kit *k)
 
 bool Project::copySteps(Target *sourceTarget, Target *newTarget)
 {
+    QTC_ASSERT(newTarget, return false);
     bool fatalError = false;
     QStringList buildconfigurationError;
     QStringList deployconfigurationError;