android不明原因报错,该怎么解决

android不明原因报错
最近，写一段代码，挺简单的，可是报了一个The application has stopped unexception.求大虾们帮助看看，怎么回事。代码如下：
first.xml
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
  android:orientation="vertical"
  android:layout_width="fill_parent"
  android:layout_height="fill_parent"
  >
<TextView  
  android:layout_width="fill_parent" 
  android:layout_height="wrap_content" 
  android:text="欢迎使用天马行空为您呈现的隐写系统：请选择目标图片"
  />
<ImageButton 
  android:layout_width="wrap_content" 
  android:src="@drawable/open" 
  android:id="@+id/ibtn1" 
  android:layout_height="wrap_content">
  </ImageButton>
</LinearLayout>


comsystem.java
package com.tmxk;

import android.app.Activity;
import android.os.Bundle;
import android.widget.ImageButton;

public class ComsystemActivity extends Activity {
  /** Called when the activity is first created. */
ImageButton btn1 = (ImageButton)findViewById(R.id.ibtn1);
  @Override
  public void onCreate(Bundle savedInstanceState) {
  super.onCreate(savedInstanceState);
  setContentView(R.layout.first);
  }
}



logcat信息为：
05-02 11:02:41.782: DEBUG/AndroidRuntime(299): >>>>>>>>>>>>>> AndroidRuntime START <<<<<<<<<<<<<<
05-02 11:02:41.782: DEBUG/AndroidRuntime(299): CheckJNI is ON
05-02 11:02:41.991: DEBUG/AndroidRuntime(299): --- registering native functions ---
05-02 11:02:42.302: DEBUG/ddm-heap(299): Got feature list request
05-02 11:02:42.621: DEBUG/AndroidRuntime(299): Shutting down VM
05-02 11:02:42.631: DEBUG/dalvikvm(299): DestroyJavaVM waiting for non-daemon threads to exit
05-02 11:02:42.631: DEBUG/dalvikvm(299): DestroyJavaVM shutting VM down
05-02 11:02:42.631: DEBUG/dalvikvm(299): HeapWorker thread shutting down
05-02 11:02:42.631: DEBUG/dalvikvm(299): HeapWorker thread has shut down
05-02 11:02:42.631: DEBUG/jdwp(299): JDWP shutting down net...
05-02 11:02:42.642: INFO/dalvikvm(299): Debugger has detached; object registry had 1 entries
05-02 11:02:42.642: DEBUG/dalvikvm(299): VM cleaning up
05-02 11:02:42.651: ERROR/AndroidRuntime(299): ERROR: thread attach failed
05-02 11:02:42.662: DEBUG/dalvikvm(299): LinearAlloc 0x0 used 629532 of 5242880 (12%)
05-02 11:02:43.091: DEBUG/AndroidRuntime(307): >>>>>>>>>>>>>> AndroidRuntime START <<<<<<<<<<<<<<
05-02 11:02:43.091: DEBUG/AndroidRuntime(307): CheckJNI is ON
05-02 11:02:43.271: DEBUG/AndroidRuntime(307): --- registering native functions ---
05-02 11:02:43.571: DEBUG/ddm-heap(307): Got feature list request
05-02 11:02:43.901: INFO/ActivityManager(52): Starting activity: Intent { act=android.intent.action.MAIN cat=[android.intent.category.LAUNCHER] flg=0x10000000 cmp=com.tmxk/.ComsystemActivity }
05-02 11:02:43.971: DEBUG/AndroidRuntime(307): Shutting down VM
05-02 11:02:43.971: DEBUG/dalvikvm(307): DestroyJavaVM waiting for non-daemon threads to exit
05-02 11:02:43.971: DEBUG/dalvikvm(307): DestroyJavaVM shutting VM down
05-02 11:02:43.971: DEBUG/dalvikvm(307): HeapWorker thread shutting down
05-02 11:02:43.971: DEBUG/dalvikvm(307): HeapWorker thread has shut down
05-02 11:02:43.971: DEBUG/jdwp(307): JDWP shutting down net...
05-02 11:02:43.981: INFO/dalvikvm(307): Debugger has detached; object registry had 1 entries
05-02 11:02:43.981: DEBUG/dalvikvm(307): VM cleaning up