debugger: fix crash on "recheck debugging helper" when not debugging

src/plugins/debugger/debuggermanager.cpp

@@ -1533,7 +1533,7 @@ void DebuggerManager::showQtDumperLibraryWarning(const QString &details)
 
 DebuggerStartMode DebuggerManager::startMode() const
 {
-    return m_runControl->startMode();
+    return m_runControl ? m_runControl->startMode() : NoStartMode;
 }
 
 void DebuggerManager::reloadFullStack()

src/plugins/debugger/debuggermanager.h

@@ -118,6 +118,7 @@ enum DebuggerStatus
 
 enum DebuggerStartMode
 {
+    NoStartMode,
     StartInternal,         // Start current start project's binary
     StartExternal,         // Start binary found in file system
     AttachExternal,        // Attach to running process by process id