ProjectExplorer: make sure there is no nullptr buildConfiguration

This caused a crash in QtDesinStudio calling the preview in buildmanager.cpp:

  for (const BuildConfiguration * const bc
    : buildConfigsForSelection(target, configSelection)) {
        bsl = isBuild ? bc->buildSteps() : bc->cleanSteps(); //bc was nullptr here

Change-Id: I67d165adb1b81c1b68fbc6fb38b67d4d2887eed1
Reviewed-by: hjk <hjk@qt.io>
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>

src/plugins/projectexplorer/buildmanager.cpp

@@ -81,7 +81,9 @@ static const QList<BuildConfiguration *> buildConfigsForSelection(const Target *
 {
     if (configSelection == ConfigSelection::All)
         return target->buildConfigurations();
-    return {target->activeBuildConfiguration()};
+    else if (target->activeBuildConfiguration())
+        return {target->activeBuildConfiguration()};
+    return {};
 }
 
 static int queue(const QList<Project *> &projects, const QList<Id> &stepIds,