webview_flutter Route fallback ,the application crashed.

[XHL-study]

When I stepped back the route, the application crashed.
console log

E/AndroidRuntime( 5177): java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.inputmethod.InputConnection android.view.View.onCreateInputConnection(android.view.inputmethod.EditorInfo)' on a null object reference
E/AndroidRuntime( 5177): 	at io.flutter.plugin.editing.TextInputPlugin.createInputConnection(TextInputPlugin.java:195)
E/AndroidRuntime( 5177): 	at io.flutter.view.FlutterView.onCreateInputConnection(FlutterView.java:397)
E/AndroidRuntime( 5177): 	at android.view.inputmethod.InputMethodManager.startInputInner(InputMethodManager.java:1236)
E/AndroidRuntime( 5177): 	at android.view.inputmethod.InputMethodManager.checkFocus(InputMethodManager.java:1428)
E/AndroidRuntime( 5177): 	at android.view.inputmethod.InputMethodManager.isActive(InputMethodManager.java:786)
E/AndroidRuntime( 5177): 	at io.flutter.plugins.webviewflutter.InputAwareWebView$1.run(InputAwareWebView.java:153)
E/AndroidRuntime( 5177): 	at android.os.Handler.handleCallback(Handler.java:815)
E/AndroidRuntime( 5177): 	at android.os.Handler.dispatchMessage(Handler.java:104)
E/AndroidRuntime( 5177): 	at android.os.Looper.loop(Looper.java:207)
E/AndroidRuntime( 5177): 	at android.app.ActivityThread.main(ActivityThread.java:5845)
E/AndroidRuntime( 5177): 	at java.lang.reflect.Method.invoke(Native Method)
E/AndroidRuntime( 5177): 	at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:907)
E/AndroidRuntime( 5177): 	at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:768)
I/BufferQueueProducer( 5177): [SurfaceTexture-0-5177-0](this:0x7f5a47c400,id:0,api:1,p:-1,c:-1) disconnect(P): api 1
I/BufferQueueProducer( 5177): [SurfaceTexture-0-5177-0](this:0x7f5a47c400,id:0,api:1,p:-1,c:-1) disconnect(P): api 1
I/Process ( 5177): Sending signal. PID: 5177 SIG: 9
Lost connection to device.

webview_flutter: ^0.3.10+2

flutter doctor

Doctor summary (to see all details, run flutter doctor -v):
[√] Flutter (Channel stable, v1.7.8+hotfix.3, on Microsoft Windows [Version 10.0.17134.829], locale zh-CN)

[√] Android toolchain - develop for Android devices (Android SDK version 29.0.0-rc2)
[√] Android Studio (version 3.4)
[!] IntelliJ IDEA Ultimate Edition (version 2019.1)
    X Flutter plugin not installed; this adds Flutter specific functionality.
    X Dart plugin not installed; this adds Dart specific functionality.
[√] Connected device (1 available)

! Doctor found issues in 1 category.

[mklim]

I'm seeing this bug on stable, master looks unaffected.

Following mklim/plugins@3e033be the crash happens on step 4.

This is a little tricky because the obvious solution--guarding against the null exception--isn't going to actually help here, since the exception is in the engine. So the fix would be stuck in master too. Looking to see if I can solve this from the plugin side. It seems like there's some state between the two that's gotten out of sync in the engine without the lock() and unlock() mechanism that's currently only in master.

[mklim]

The problem flow looks like:

• Part of our trick of resetting the InputConnection back to the normal thread in setInputConnectionTarget necessarily ends up calling FlutterView#onCreateInputConnection. The full flow is IMM#isActive(View) -> IMM#checkFocusNoStartInput -> IMM#startInputInner. This happens on both master and stable. The view creation happens as long as IMM#mNextServedView != IMM#mServedView, which it is in both cases here. Those are FlutterView and our ThreadedInputProxyAdapterView, respectively.
• FlutterView#onCreateInputConnection falls through to TextInputPlugin#onCreateInputConnection, which uses a cached InputTarget. We don't update the InputTarget to be anything other than the PlatformView at this point on stable, so we try to use it even though it's been disposed. So then there's the NPE.

The bug is that the engine aren't clearing out its inputTarget state correctly when PlatformViews are disposed on stable. It looks like this has already been fixed on master by @amirh three weeks ago, but the fix still hasn't rolled out yet: flutter/engine#9423.

I'm not sure of a good (or any) way to work around this in the plugin for the current Flutter stable. The issue is that the engine is holding a bad reference to some previously existing state with the platform views. I don't think we can avoid this on the plugin side other than maybe deliberately failing to set any kind of InputConnection correctly in the first place, so there's no InputConnection state at all to track. This would regress what little InputConnection support there is for stable right now but is probably preferable to crashes. @amirh what do you think?

[mklim]

Discussed offline. Given that the keyboard is really only supported on master right now to begin with it's probably better for us to leave the code as-is and wait for everything to roll. If this is an ongoing issue we could add something to completely disable the keyboard on unsupported Flutter versions like this one, but for now we'll plan on updating the docs again with a specific version number once the changes in master roll to dev.

[XHL-study]

Can you tell me when does webview support the clipboard feature?
@mklim

[mklim]

@XHL-study that's being tracked in #24585.