首先，我已经阅读了与我类似的问题，并尝试了所有推荐的解决方案。这些解决方案包括：
- 使缓存无效并重新启动
- 与 gradle 同步
- 重启 Android Studio
- 清理和构建
- 在启动器部分将配置更改为“无”
- 将清单文件上的 android:name 更改为完整的包名（例如：com.example.acer.alertbox.ActivityName）
这些解决方案似乎都不起作用，并且我不断收到错误消息说“未找到默认活动”。我已经尝试解决这个问题一个星期了，所以我希望有人能给我一个新的解决方案，除了我上面提到的 6 个。下面是我的 android 清单代码，对我来说似乎很好，并且我已经检查过多次。
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
package="com.example.acer.alertbox">
<uses-permission android:name="android.permission.INTERNET"/>
<uses-permission android:name="android.permission.READ_PHONE_STATE"/>
<application
android:allowBackup="true"
android:icon="@mipmap/ic_launcher"
android:label="@string/app_name"
android:roundIcon="@mipmap/ic_launcher_round"
android:supportsRtl="true"
android:theme="@style/AppTheme"
tools:ignore="GoogleAppIndexingWarning">
<activity android:name=".MainActivity">
<intent-filter>
<action android:name="android.intent.action.MAIN"/>
<category android:name="android.intent.category.LAUNCHER"/>
</intent-filter>
</activity>
<activity
android:name=".LoginActivity"
android:parentActivityName=".MainActivity">
</activity>
<activity
android:name=".RegisterActivity"
android:parentActivityName=".MainActivity">
</activity>
<activity
android:name=".SendActivity"
android:parentActivityName=".MainActivity">
</activity>
<activity
android:name=".Profile"
android:parentActivityName=".MainActivity">
</activity>
<activity android:name=".NotificationActivity">
<intent-filter>
<action android:name="com.example.acer.alertbox.TARGETNOTIFICATION">
</action>
</intent-filter>
</activity>
<service
android:name=".FirebaseMessagingService">
<intent-filter>
<action android:name="com.google.firebase.MESSAGING_EVENT">
</action>
</intent-filter>
</service>
<meta-data
android:name="com.google.firebase.messaging.default_notification_channel_id"
android:value="@string/default_notification_channel_id" />
</application>
</manifest>
更新：我已经卸载并重新安装了Android Studio，问题解决了，但又来了一个。当我单击“运行”按钮时，它没有做任何事情，并且我在右下角收到警告说内部 IDE 错误：无法找到 adb。我已经下载了 adb 文件并将其定位在这个目录 C:/Acer/Users/Local/Sdk 但它仍然没有改变任何东西。我也重新启动，同步项目 gradle 文件等，但没有任何效果！！请有人帮助我。以下是我从内部错误报告中得到的信息：
Unable to locate adb
java.lang.IllegalArgumentException: Unable to locate adb
at com.android.tools.idea.run.editor.DeployTargetPickerDialog.<init>(DeployTargetPickerDialog.java:131)
at com.android.tools.idea.run.editor.ShowChooserTargetProvider.showPrompt(ShowChooserTargetProvider.java:113)
at com.android.tools.idea.run.AndroidRunConfigurationBase.getDeployTarget(AndroidRunConfigurationBase.java:605)
at com.android.tools.idea.run.AndroidRunConfigurationBase.getState(AndroidRunConfigurationBase.java:287)
at com.intellij.execution.runners.ExecutionEnvironment.getState(ExecutionEnvironment.java:158)
at com.intellij.execution.runners.BaseProgramRunner.execute(BaseProgramRunner.java:55)
at com.intellij.execution.runners.BaseProgramRunner.execute(BaseProgramRunner.java:50)
at com.intellij.execution.ProgramRunnerUtil.executeConfigurationAsync(ProgramRunnerUtil.java:97)
at com.intellij.execution.ProgramRunnerUtil.executeConfiguration(ProgramRunnerUtil.java:44)
at com.intellij.execution.impl.ExecutionManagerImpl.start(ExecutionManagerImpl.java:111)
at com.intellij.execution.impl.ExecutionManagerImpl.access$300(ExecutionManagerImpl.java:59)
at com.intellij.execution.impl.ExecutionManagerImpl$3.run(ExecutionManagerImpl.java:439)
at com.intellij.util.concurrency.QueueProcessor.runSafely(QueueProcessor.java:231)
at com.intellij.util.Alarm$Request.runSafely(Alarm.java:405)
at com.intellij.util.Alarm$Request.access$700(Alarm.java:330)
at com.intellij.util.Alarm$Request$1.run(Alarm.java:371)
at com.intellij.openapi.application.TransactionGuardImpl$2.run(TransactionGuardImpl.java:315)
at com.intellij.openapi.application.impl.LaterInvocator$FlushQueue.doRun(LaterInvocator.java:447)
at com.intellij.openapi.application.impl.LaterInvocator$FlushQueue.runNextEvent(LaterInvocator.java:431)
at com.intellij.openapi.application.impl.LaterInvocator$FlushQueue.run(LaterInvocator.java:415)
at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:311)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:762)
at java.awt.EventQueue.access$500(EventQueue.java:98)
at java.awt.EventQueue$3.run(EventQueue.java:715)
at java.awt.EventQueue$3.run(EventQueue.java:709)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:80)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:732)
at com.intellij.ide.IdeEventQueue.defaultDispatchEvent(IdeEventQueue.java:822)
at com.intellij.ide.IdeEventQueue._dispatchEvent(IdeEventQueue.java:763)
at com.intellij.ide.IdeEventQueue.dispatchEvent(IdeEventQueue.java:423)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:201)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:116)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:105)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:93)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:82)
最新更新：通过从 Avast 防病毒软件恢复 adb.exe 解决了问题。