关闭键盘时，会报这个错：Null check operator used on a null value

[yuping1989]

关闭键盘时，会报这个错：Null check operator used on a null value

════════ Exception caught by widgets library ═══════════════════════════════════
The following _CastError was thrown building Builder(dirty):
Null check operator used on a null value

The relevant error-causing widget was
KeyboardRootWidget
lib/main.dart:93
When the exception was thrown, this was the stack
#0 CoolKeyboard.openKeyboard..
package:cool_ui/keyboards/keyboard_manager.dart:173
#1 KeyboardPageState.build.
package:cool_ui/keyboards/keyboard_manager.dart:415
#2 Builder.build
package:flutter/…/widgets/basic.dart:7555
#3 StatelessElement.build
package:flutter/…/widgets/framework.dart:4569
#4 ComponentElement.performRebuild
package:flutter/…/widgets/framework.dart:4495
...
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