For #26723 - Fix WallpapersObserver leaking a view from HomeFragment

The observer initialized with a fragment View would outlive the fragment and in
that allow for leaking the entire fragment layout.

app/src/main/java/org/mozilla/fenix/home/HomeFragment.kt

@@ -121,7 +121,8 @@ import kotlin.math.min
 @Suppress("TooManyFunctions", "LargeClass")
 class HomeFragment : Fragment() {
     private val args by navArgs<HomeFragmentArgs>()
-    private lateinit var bundleArgs: Bundle
+    @VisibleForTesting
+    internal lateinit var bundleArgs: Bundle
 
     private var _binding: FragmentHomeBinding? = null
     private val binding get() = _binding!!
@@ -163,7 +164,7 @@ class HomeFragment : Fragment() {
     private var appBarLayout: AppBarLayout? = null
     private lateinit var currentMode: CurrentMode
     @VisibleForTesting
-    internal lateinit var wallpapersObserver: WallpapersObserver
+    internal var wallpapersObserver: WallpapersObserver? = null
 
     private val topSitesFeature = ViewBoundFeatureWrapper<TopSitesFeature>()
     private val messagingFeature = ViewBoundFeatureWrapper<MessagingFeature>()
@@ -418,7 +419,7 @@ class HomeFragment : Fragment() {
             // Otherwise the portrait wallpaper may remain shown on landscape,
             // see https://github.com/mozilla-mobile/fenix/issues/26638
             runBlockingIncrement {
-                wallpapersObserver.applyCurrentWallpaper()
+                wallpapersObserver?.applyCurrentWallpaper()
             }
         }
     }
@@ -700,6 +701,7 @@ class HomeFragment : Fragment() {
         _sessionControlInteractor = null
         sessionControlView = null
         appBarLayout = null
+        wallpapersObserver = null
         _binding = null
         bundleArgs.clear()
     }

app/src/test/java/org/mozilla/fenix/home/HomeFragmentTest.kt

@@ -19,6 +19,7 @@ import mozilla.components.feature.top.sites.TopSite
 import org.junit.Assert.assertEquals
 import org.junit.Assert.assertFalse
 import org.junit.Assert.assertNotNull
+import org.junit.Assert.assertNull
 import org.junit.Assert.assertTrue
 import org.junit.Before
 import org.junit.Test
@@ -156,4 +157,14 @@ class HomeFragmentTest {
 
         assertFalse(homeFragment.shouldEnableWallpaper())
     }
+
+    @Test
+    fun `GIVEN the wallpaper feature is active WHEN the fragment view is destroyed THEN cleanup the wallpaper observer`() {
+        homeFragment.bundleArgs = mockk(relaxed = true)
+        homeFragment.wallpapersObserver = mockk()
+
+        homeFragment.onDestroyView()
+
+        assertNull(homeFragment.wallpapersObserver)
+    }
 }