无法在微调器上显示数据？抛出空指针异常

任何人都可以告诉这个代码中的错误...我把代码放在下面 .

mySQLiteAdapter = new SQLiteAdapter(this);
    cursor = mySQLiteAdapter.displayCategory();
    startManagingCursor(cursor);

    String[] from = new String[] { SQLiteAdapter.KEY_ID, SQLiteAdapter.KEY_CATEGORY };

    int[] to = new int[] { android.R.id.text1 };

    mCategory = (Spinner) findViewById(R.id.choose_category);
    SimpleCursorAdapter adapter = new SimpleCursorAdapter(this,
            android.R.layout.simple_spinner_item, cursor, from, to);
    adapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
    mCategory.setAdapter(adapter);

MyAdapter类有以下代码

public Cursor displayCategory() {
        String[] columns = new String[]{KEY_ID, KEY_CATEGORY};          
        Cursor cursor = sqLiteDatabase.query(MYCATEGORY_TABLE, columns, 
                null, null, null, null, null);

        return cursor;

空指针异常 . 如何避免它 . 这是log cat的错误

09-09 15:49:16.535: ERROR/AndroidRuntime(6526): Uncaught handler: thread main exiting  due to uncaught exception
09-09 15:49:16.545: ERROR/AndroidRuntime(6526): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.android.mpt.receiptor/com.android.mpt.receiptor.SearchReceipt}: java.lang.NullPointerException
09-09 15:49:16.545: ERROR/AndroidRuntime(6526):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2496)
09-09 15:49:16.545: ERROR/AndroidRuntime(6526):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2512)
09-09 15:49:16.545: ERROR/AndroidRuntime(6526):     at android.app.ActivityThread.access$2200(ActivityThread.java:119)
09-09 15:49:16.545: ERROR/AndroidRuntime(6526):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1863)
09-09 15:49:16.545: ERROR/AndroidRuntime(6526):     at android.os.Handler.dispatchMessage(Handler.java:99)
09-09 15:49:16.545: ERROR/AndroidRuntime(6526):     at android.os.Looper.loop(Looper.java:123)
09-09 15:49:16.545: ERROR/AndroidRuntime(6526):     at android.app.ActivityThread.main(ActivityThread.java:4363)
09-09 15:49:16.545: ERROR/AndroidRuntime(6526):     at java.lang.reflect.Method.invokeNative(Native Method)
09-09 15:49:16.545: ERROR/AndroidRuntime(6526):     at java.lang.reflect.Method.invoke(Method.java:521)
09-09 15:49:16.545: ERROR/AndroidRuntime(6526):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:860)
09-09 15:49:16.545: ERROR/AndroidRuntime(6526):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:618)
09-09 15:49:16.545: ERROR/AndroidRuntime(6526):     at dalvik.system.NativeStart.main(Native Method)
09-09 15:49:16.545: ERROR/AndroidRuntime(6526): Caused by: java.lang.NullPointerException
09-09 15:49:16.545: ERROR/AndroidRuntime(6526):     at com.android.mpt.receiptor.SQLiteAdapter.displayCategory(SQLiteAdapter.java:135)
09-09 15:49:16.545: ERROR/AndroidRuntime(6526):     at com.android.mpt.receiptor.SearchReceipt.onCreate(SearchReceipt.java:96)
09-09 15:49:16.545: ERROR/AndroidRuntime(6526):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)
09-09 15:49:16.545: ERROR/AndroidRuntime(6526):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2459)

请别人帮助我 .

提前致谢