diff --git a/CHANGELOG.md b/CHANGELOG.md
index b9fca17..e31037f 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -31,6 +31,7 @@
   * Needs "Countly.UI.WebView2" for displaying content on WPF.
 * Fixed a bug where the request queue was not processed when automatic user property saving ("autoSendUserDetails") was disabled, so queued requests (for example from a manual session update) were never uploaded and could cause the SDK to loop indefinitely.
 * Fixed a bug where a user-details change that serialized to nothing (empty user details) left the internal "changed" flag set forever, so the SDK kept re-processing user details on every session call and upload-completion checks could wait indefinitely.
+* Fixed a crash ("NullReferenceException") when a session-timer tick landed while/after "Halt" cleared the SDK state (an unhandled exception on a timer thread can terminate the host application), and when "SessionUpdate" was called before the first "Init".
 
 ## 25.4.3
 * ! Minor breaking change ! User properties will now be automatically saved under the following conditions:
diff --git a/countlyCommon/TestingRelated/SessionTests.cs b/countlyCommon/TestingRelated/SessionTests.cs
index d4ca590..6686cc1 100644
--- a/countlyCommon/TestingRelated/SessionTests.cs
+++ b/countlyCommon/TestingRelated/SessionTests.cs
@@ -31,6 +31,31 @@ public void Dispose()
 
         }
 
+        [Fact]
+        /// <summary>
+        /// A session-timer tick that lands after Halt must not throw: Halt nulls the modules while
+        /// the old Configuration object (here with backend mode on) stays visible to the in-flight tick.
+        /// </summary>
+        public void UpdateSession_TimerTickAfterHalt_DoesNotThrow()
+        {
+            CountlyConfig cc = TestHelper.GetConfig();
+            cc.EnableBackendMode();
+            Countly.Instance.Init(cc).Wait();
+            Countly.Instance.HaltInternal().Wait();
+
+            // the body the session timer runs; used to NRE on the nulled moduleBackendMode
+            Countly.Instance.UpdateSessionInternal().Wait();
+        }
+
+        [Fact]
+        /// <summary>Calling SessionUpdate before the first Init (Configuration is null) must not throw.</summary>
+        public void SessionUpdate_BeforeInit_DoesNotThrow()
+        {
+            Countly.Instance.Configuration = null; // what a fresh, never-initialized process looks like
+
+            Countly.Instance.SessionUpdate(10).Wait();
+        }
+
         private void ValidateSessionRequestParams(NameValueCollection collection, string appKey, string deviceId, string deviceIdType)
         {
             Assert.False(string.IsNullOrEmpty(collection.Get("sdk_version")));
diff --git a/countlyCommon/countlyCommon/CountlyBase.cs b/countlyCommon/countlyCommon/CountlyBase.cs
index c2dd897..e08a8a9 100644
--- a/countlyCommon/countlyCommon/CountlyBase.cs
+++ b/countlyCommon/countlyCommon/CountlyBase.cs
@@ -215,12 +215,24 @@ internal async Task<bool> SaveStoredRequests()
             }
         }
 
-        protected async Task UpdateSessionInternal(int? elapsedTime = null)
+        internal async Task UpdateSessionInternal(int? elapsedTime = null)
         {
             UtilityHelper.CountlyLogging("[CountlyBase] Session Update happening'");
-            if (Configuration.backendMode) {
-                moduleBackendMode.OnTimer();
-                Upload();
+
+            // A session-timer tick can land while/after Halt nulls the SDK state. Work on local
+            // captures and bail out on a halted or never-initialized SDK instead of crashing.
+            CountlyConfig config = Configuration;
+            if (config == null || !IsInitialized()) {
+                UtilityHelper.CountlyLogging("[CountlyBase] UpdateSessionInternal, SDK is not initialized (or halted), ignoring session update");
+                return;
+            }
+
+            if (config.backendMode) {
+                ModuleBackendMode backendMode = moduleBackendMode;
+                if (backendMode != null) {
+                    backendMode.OnTimer();
+                    Upload();
+                }
                 return;
             }
 
@@ -1760,14 +1772,16 @@ public async Task SessionBegin()
         /// <returns></returns>
         public async Task SessionUpdate(int elapsedTimeSeconds)
         {
-            if (Configuration.backendMode) {
-                UtilityHelper.CountlyLogging("[CountlyBase] SessionUpdate, Backend Mode enabled, returning");
+            UtilityHelper.CountlyLogging("[CountlyBase] Calling 'SessionUpdate'");
+            // The initialization check must come first: before the first Init (or after Halt)
+            // there is no Configuration object to read the backend-mode flag from.
+            if (!IsInitialized() || Configuration == null) {
+                UtilityHelper.CountlyLogging("[CountlyBase] SessionUpdate: SDK must initialized before calling 'SessionUpdate'");
                 return;
             }
 
-            UtilityHelper.CountlyLogging("[CountlyBase] Calling 'SessionUpdate'");
-            if (!IsInitialized()) {
-                UtilityHelper.CountlyLogging("[CountlyBase] SessionUpdate: SDK must initialized before calling 'SessionUpdate'");
+            if (Configuration.backendMode) {
+                UtilityHelper.CountlyLogging("[CountlyBase] SessionUpdate, Backend Mode enabled, returning");
                 return;
             }
             if (elapsedTimeSeconds < 0) {