您好老师!为什么我自定义完了出现空指针的异常 java.lang.NullPointerException    at android.graphics.Canvas.throwIfCannotDraw(Canvas.java:1224)    at android.graphics.Canvas.drawBitmap(Canvas.java:1359)    at com.imooc.weixin6_0.ChangeColorIconWithText.onDraw(ChangeColorIconWithText.java:117)