QmakePM: Avoid calling function on nullptr

Task-number: QTCREATORBUG-19419
Change-Id: Iaac597b947ca505819674eec0a862158dce8a632
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>

src/plugins/qmakeprojectmanager/qmakeproject.cpp

@@ -668,6 +668,7 @@ QtSupport::ProFileReader *QmakeProject::createProFileReader(const QmakeProFile *
         } else {
             // Set up a better default environment without using a build configuration:
             QmakeBuildConfiguration::setupBuildEnvironment(k, env);
+            if (k)
                 k->addToEnvironment(env);
         }