fix: deadlock and use-after-free in tests (#4981)

* fix: use-after-free in settings

* refactor: put seventv api into a singleton

* chore: add changelog entry

* Add warning for when the 7TV load fails

src/singletons/Settings.cpp

@@ -137,6 +137,7 @@ bool Settings::toggleMutedChannel(const QString &channelName)
 Settings *Settings::instance_ = nullptr;
 
 Settings::Settings(const QString &settingsDirectory)
+    : prevInstance_(Settings::instance_)
 {
     QString settingsPath = settingsDirectory + "/settings.json";
 
@@ -188,7 +189,10 @@ Settings::Settings(const QString &settingsDirectory)
         false);
 }
 
-Settings::~Settings() = default;
+Settings::~Settings()
+{
+    Settings::instance_ = this->prevInstance_;
+}
 
 void Settings::saveSnapshot()
 {