Merge pull request #1728 from android/trambui/navigation-null-debug

incorporate safe fallback destination to fix null destination causing UI flickering

Author: dturner

app/src/main/kotlin/com/google/samples/apps/nowinandroid/ui/NiaAppState.kt

@@ -18,14 +18,15 @@ package com.google.samples.apps.nowinandroid.ui
 
 import androidx.compose.runtime.Composable
 import androidx.compose.runtime.Stable
+import androidx.compose.runtime.collectAsState
+import androidx.compose.runtime.mutableStateOf
 import androidx.compose.runtime.remember
 import androidx.compose.runtime.rememberCoroutineScope
 import androidx.navigation.NavController
 import androidx.navigation.NavDestination
 import androidx.navigation.NavDestination.Companion.hasRoute
 import androidx.navigation.NavGraph.Companion.findStartDestination
 import androidx.navigation.NavHostController
-import androidx.navigation.compose.currentBackStackEntryAsState
 import androidx.navigation.compose.rememberNavController
 import androidx.navigation.navOptions
 import androidx.tracing.trace
@@ -83,9 +84,21 @@ class NiaAppState(
     userNewsResourceRepository: UserNewsResourceRepository,
     timeZoneMonitor: TimeZoneMonitor,
 ) {
+    private val previousDestination = mutableStateOf<NavDestination?>(null)
+
     val currentDestination: NavDestination?
-        @Composable get() = navController
-            .currentBackStackEntryAsState().value?.destination
+        @Composable get() {
+            // Collect the currentBackStackEntryFlow as a state
+            val currentEntry = navController.currentBackStackEntryFlow
+                .collectAsState(initial = null)
+
+            // Fallback to previousDestination if currentEntry is null
+            return currentEntry.value?.destination.also { destination ->
+                if (destination != null) {
+                    previousDestination.value = destination
+                }
+            } ?: previousDestination.value
+        }
 
     val currentTopLevelDestination: TopLevelDestination?
         @Composable get() {