controllers: Fix crash if controller mapping has no module

This fixes two bugs that were caused by mixxxdj#2868:

    1. Mixxx didn't check if the JS module path was really a file. If
       you pass in a directory, QtJSEngine::importModule makes Mixxx
       crash (at least on Qt 5.15.0)
    2. If a mapping does not have JS module associated with it, then the
       moduleFileInfo always pointed to the controller directory and
       hence triggered bug 1.

This fixes both of these issues and adds some debug assertions to make
sure it stays like that.

src/controllers/controller.cpp

@@ -74,7 +74,11 @@ bool Controller::applyPreset(bool initializeScripts) {
         m_pEngine->initializeScripts(scriptFiles);
     }
 
-    if (initializeScripts) {
+    // QFileInfo does not have a isValid/isEmpty/isNull method to check if it
+    // actually contains a reference, so we check if the filePath is empty as a
+    // workaround.
+    // See https://stackoverflow.com/a/45652741/1455128 for details.
+    if (initializeScripts && !pPreset->moduleFileInfo().filePath().isEmpty()) {
         m_pEngine->loadModule(pPreset->moduleFileInfo());
     }
     return success;

src/controllers/controllerpresetfilehandler.cpp

@@ -146,7 +146,18 @@ void ControllerPresetFileHandler::addScriptFilesToPreset(
     }
 
     QString moduleFileName = controller.firstChildElement("module").text();
-    preset->setModuleFileInfo(preset->dirPath().absoluteFilePath(moduleFileName));
+
+    if (moduleFileName.isEmpty()) {
+        return;
+    }
+
+    QFileInfo moduleFileInfo(preset->dirPath().absoluteFilePath(moduleFileName));
+    if (!moduleFileInfo.isFile()) {
+        qWarning() << "Controller Module is not a file:" << moduleFileInfo.absoluteFilePath();
+        return;
+    }
+
+    preset->setModuleFileInfo(moduleFileInfo);
 }
 
 bool ControllerPresetFileHandler::writeDocument(

src/controllers/engine/controllerengine.cpp

@@ -230,6 +230,17 @@ void ControllerEngine::uninitializeScriptEngine() {
 }
 
 void ControllerEngine::loadModule(QFileInfo moduleFileInfo) {
+    // QFileInfo does not have a isValid/isEmpty/isNull method to check if it
+    // actually contains a reference, so we check if the filePath is empty as a
+    // workaround.
+    // See https://stackoverflow.com/a/45652741/1455128 for details.
+    VERIFY_OR_DEBUG_ASSERT(!moduleFileInfo.filePath().isEmpty()) {
+        return;
+    }
+
+    VERIFY_OR_DEBUG_ASSERT(moduleFileInfo.isFile()) {
+        return;
+    }
 #if QT_VERSION >= QT_VERSION_CHECK(5, 12, 0)
     m_moduleFileInfo = moduleFileInfo;
 
@@ -318,7 +329,14 @@ void ControllerEngine::reloadScripts() {
 
     qDebug() << "Re-initializing scripts";
     initializeScripts(m_lastScriptFiles);
-    loadModule(m_moduleFileInfo);
+
+    // QFileInfo does not have a isValid/isEmpty/isNull method to check if it
+    // actually contains a reference, so we check if the filePath is empty as a
+    // workaround.
+    // See https://stackoverflow.com/a/45652741/1455128 for details.
+    if (!m_moduleFileInfo.filePath().isEmpty()) {
+        loadModule(m_moduleFileInfo);
+    }
 }
 
 void ControllerEngine::initializeScripts(const QList<ControllerPreset::ScriptFileInfo>& scripts) {