Fix crash in pre-launch logging.

Fixes #6635.

Telegram/SourceFiles/ui/ui_integration.cpp

@@ -27,12 +27,16 @@ Integration &Integration::Instance() {
 	return *IntegrationInstance;
 }
 
+bool Integration::Exists() {
+	return (IntegrationInstance != nullptr);
+}
+
 void Integration::textActionsUpdated() {
 }
 
-void Integration::activationFromTopPanel() {		
+void Integration::activationFromTopPanel() {
 }
-	
+
 std::shared_ptr<ClickHandler> Integration::createLinkHandler(
 		EntityType type,
 		const QString &text,

Telegram/SourceFiles/ui/ui_integration.h

@@ -28,6 +28,7 @@ class Integration {
 public:
 	static void Set(not_null<Integration*> instance);
 	static Integration &Instance();
+	static bool Exists();
 
 	virtual void postponeCall(FnMut<void()> &&callable) = 0;
 	virtual void registerLeaveSubscription(not_null<QWidget*> widget) = 0;

Telegram/SourceFiles/ui/ui_log.cpp

@@ -12,7 +12,9 @@ https://github.com/telegramdesktop/tdesktop/blob/master/LEGAL
 namespace Ui {
 
 void WriteLogEntry(const QString &message) {
-	Integration::Instance().writeLogEntry(message);
+	if (Integration::Exists()) {
+		Integration::Instance().writeLogEntry(message);
+	}
 }
 
 } // namespace Ui