Tab manager crash fix (#4865)

Task/Issue URL:
https://app.asana.com/0/1202552961248957/1207981775109546/f
Possibly also: https://app.asana.com/0/1202552961248957/1207981775109549

### Description

This PR adds bounds checks for the tab getter in the RecyclerView
adapter. The first crash is caused by a rapid double tap on the tab
close button, which results in an invalid index being returned.

The second crash is possibly a different manifestation of the same
problem but due to a race condition it happens elsewhere. It should be
resolved by
[8bfcfbd](8bfcfbd).

### Steps to test this PR

_Duble tap_
- [x] Go to the tab manager 
- [x] Add a few tabs
- [x] Tap on a tab close button very quickly
- [x] Notice the app doesn’t crash

Author: 0nko

app/src/main/java/com/duckduckgo/app/browser/tabpreview/TabEntityDiffCallback.kt

@@ -22,7 +22,7 @@ import com.duckduckgo.app.tabs.model.TabEntity
 
 class TabEntityDiffCallback(
     private val oldList: List<TabEntity>,
-    var newList: List<TabEntity>,
+    private val newList: List<TabEntity>,
 ) : DiffUtil.Callback() {
 
     private fun areItemsTheSame(

app/src/main/java/com/duckduckgo/app/tabs/ui/TabGridItemDecorator.kt

@@ -53,14 +53,10 @@ class TabGridItemDecorator(
             val child = recyclerView.getChildAt(i)
 
             val positionInAdapter = recyclerView.getChildAdapterPosition(child)
-            if (positionInAdapter < 0) {
-                continue
-            }
-
-            val tab = adapter.getTab(positionInAdapter)
-
-            if (tab.tabId == selectedTabId) {
-                drawSelectedTabDecoration(child, canvas)
+            adapter.getTab(positionInAdapter)?.let { tab ->
+                if (tab.tabId == selectedTabId) {
+                    drawSelectedTabDecoration(child, canvas)
+                }
             }
         }
 

app/src/main/java/com/duckduckgo/app/tabs/ui/TabSwitcherActivity.kt

@@ -220,7 +220,9 @@ class TabSwitcherActivity : DuckDuckGoActivity(), TabSwitcherListener, Coroutine
 
     private fun scrollToShowCurrentTab() {
         val index = tabsAdapter.adapterPositionForTab(selectedTabId)
-        tabsRecycler.post { tabsRecycler.scrollToPosition(index) }
+        if (index != -1) {
+            tabsRecycler.post { tabsRecycler.scrollToPosition(index) }
+        }
     }
 
     private fun processCommand(command: Command) {
@@ -297,8 +299,9 @@ class TabSwitcherActivity : DuckDuckGoActivity(), TabSwitcherListener, Coroutine
     }
 
     override fun onTabDeleted(position: Int, deletedBySwipe: Boolean) {
-        val tab = tabsAdapter.getTab(position)
-        launch { viewModel.onMarkTabAsDeletable(tab, deletedBySwipe) }
+        tabsAdapter.getTab(position)?.let { tab ->
+            launch { viewModel.onMarkTabAsDeletable(tab, deletedBySwipe) }
+        }
     }
 
     override fun onTabMoved(from: Int, to: Int) {

app/src/main/java/com/duckduckgo/app/tabs/ui/TabSwitcherAdapter.kt

@@ -54,7 +54,6 @@ class TabSwitcherAdapter(
 ) : Adapter<TabViewHolder>() {
 
     private val list = mutableListOf<TabEntity>()
-    private val diffCallback = TabEntityDiffCallback(list, listOf())
 
     private var isDragging: Boolean = false
 
@@ -201,15 +200,14 @@ class TabSwitcherAdapter(
     }
 
     private fun submitList(updatedList: List<TabEntity>) {
-        diffCallback.newList = updatedList
-        val diffResult = DiffUtil.calculateDiff(diffCallback)
+        val diffResult = DiffUtil.calculateDiff(TabEntityDiffCallback(list, updatedList))
 
         list.clear()
         list.addAll(updatedList)
         diffResult.dispatchUpdatesTo(this)
     }
 
-    fun getTab(position: Int): TabEntity = list[position]
+    fun getTab(position: Int): TabEntity? = list.getOrNull(position)
 
     fun adapterPositionForTab(tabId: String?): Int {
         if (tabId == null) return -1

app/src/main/java/com/duckduckgo/app/tabs/ui/TabSwitcherViewModel.kt

@@ -55,9 +55,9 @@ class TabSwitcherViewModel @Inject constructor(
         const val REINSTALL_VARIANT = "ru"
     }
 
-    var tabs: LiveData<List<TabEntity>> = tabRepository.liveTabs
+    val tabs: LiveData<List<TabEntity>> = tabRepository.liveTabs
     val activeTab = tabRepository.liveSelectedTab
-    var deletableTabs: LiveData<List<TabEntity>> = tabRepository.flowDeletableTabs.asLiveData(
+    val deletableTabs: LiveData<List<TabEntity>> = tabRepository.flowDeletableTabs.asLiveData(
         context = viewModelScope.coroutineContext,
     )