Fix: cancel coroutine when activity gets destroyed (duckduckgo#699)

* Cancel coroutine running on activity context when activity gets stopped.

* remove fragment reference when activity gets destroyed.

Author: cmonfortep

app/src/main/java/com/duckduckgo/app/browser/BrowserActivity.kt

@@ -56,7 +56,7 @@ import kotlin.coroutines.CoroutineContext
 class BrowserActivity : DuckDuckGoActivity(), CoroutineScope {
 
     override val coroutineContext: CoroutineContext
-        get() = SupervisorJob() + Dispatchers.Main
+        get() = job + Dispatchers.Main
 
     @Inject
     lateinit var clearPersonalDataAction: ClearPersonalDataAction
@@ -70,6 +70,8 @@ class BrowserActivity : DuckDuckGoActivity(), CoroutineScope {
     @Inject
     lateinit var playStoreUtils: PlayStoreUtils
 
+    private val job = SupervisorJob()
+
     private var currentTab: BrowserTabFragment? = null
 
     private val viewModel: BrowserViewModel by bindViewModel()
@@ -101,6 +103,16 @@ class BrowserActivity : DuckDuckGoActivity(), CoroutineScope {
         viewModel.awaitClearDataFinishedNotification()
     }
 
+    override fun onStop() {
+        job.cancel()
+        super.onStop()
+    }
+
+    override fun onDestroy() {
+        currentTab = null
+        super.onDestroy()
+    }
+
     override fun onNewIntent(intent: Intent?) {
         super.onNewIntent(intent)
         Timber.i("onNewIntent: $intent")