diff --git a/build.gradle b/build.gradle
index c13e4f32..afa4e4a9 100644
--- a/build.gradle
+++ b/build.gradle
@@ -1,18 +1,21 @@
 buildscript {
     repositories {
+        maven {
+            url "https://plugins.gradle.org/m2/"
+        }
         jcenter()
         google()
     }
     dependencies {
-        classpath 'com.android.tools.build:gradle:3.3.1'
+        classpath 'com.android.tools.build:gradle:3.4.1'
         classpath 'com.jfrog.bintray.gradle:gradle-bintray-plugin:1.7.3'
     }
 }
 
 allprojects {
     repositories {
-        jcenter()
         google()
+        jcenter()
     }
 }
 
diff --git a/gradle/wrapper/gradle-wrapper.properties b/gradle/wrapper/gradle-wrapper.properties
index a58200fb..b5f789de 100644
--- a/gradle/wrapper/gradle-wrapper.properties
+++ b/gradle/wrapper/gradle-wrapper.properties
@@ -3,4 +3,4 @@ distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
 zipStoreBase=GRADLE_USER_HOME
 zipStorePath=wrapper/dists
-distributionUrl=https\://services.gradle.org/distributions/gradle-4.10.1-all.zip
+distributionUrl=https\://services.gradle.org/distributions/gradle-5.4.1-all.zip
diff --git a/library/src/main/java/com/liulishuo/filedownloader/FileDownloadMessenger.java b/library/src/main/java/com/liulishuo/filedownloader/FileDownloadMessenger.java
index 80fce2f0..d5742597 100644
--- a/library/src/main/java/com/liulishuo/filedownloader/FileDownloadMessenger.java
+++ b/library/src/main/java/com/liulishuo/filedownloader/FileDownloadMessenger.java
@@ -103,6 +103,13 @@ public void notifyConnected(MessageSnapshot snapshot) {
 
     @Override
     public void notifyProgress(MessageSnapshot snapshot) {
+        if (mTask == null) {
+            if (FileDownloadLog.NEED_LOG) {
+                FileDownloadLog.d(this, "can't notify progress snapshot(id[%d], status[%d])",
+                        snapshot.getId(), snapshot.getStatus());
+            }
+            return;
+        }
         final BaseDownloadTask originTask = mTask.getOrigin();
         if (FileDownloadLog.NEED_LOG) {
             FileDownloadLog.d(this, "notify progress %s %d %d",
@@ -139,6 +146,13 @@ public void notifyBlockComplete(MessageSnapshot snapshot) {
 
     @Override
     public void notifyRetry(MessageSnapshot snapshot) {
+        if (mTask == null) {
+            if (FileDownloadLog.NEED_LOG) {
+                FileDownloadLog.d(this, "can't notify retry snapshot(id[%d], status[%d])",
+                        snapshot.getId(), snapshot.getStatus());
+            }
+            return;
+        }
         if (FileDownloadLog.NEED_LOG) {
             final BaseDownloadTask originTask = mTask.getOrigin();
             FileDownloadLog.d(this, "notify retry %s %d %d %s", mTask,
@@ -165,6 +179,13 @@ public void notifyWarn(MessageSnapshot snapshot) {
 
     @Override
     public void notifyError(MessageSnapshot snapshot) {
+        if (mTask == null) {
+            if (FileDownloadLog.NEED_LOG) {
+                FileDownloadLog.d(this, "can't notify error snapshot(id[%d], status[%d])",
+                        snapshot.getId(), snapshot.getStatus());
+            }
+            return;
+        }
         if (FileDownloadLog.NEED_LOG) {
             FileDownloadLog.d(this, "notify error %s %s", mTask, mTask.getOrigin().getErrorCause());
         }