Remove observables callbacks when kernel changes (fixes #)

With the notebook

```julia
using PlotlyJS, Observables
```

```julia
p = plot(scatter(;y=rand(20))
```

```julia
on(p["hover"]) do hover
    @show hover
end
```

Run each cell, hover over several points to see everything working. There should be no errors in the browser console related to hover events.

Restart the kernel, and then hover over points again. There should be errors for every hover event.

Author: halleysfifthinc

webio_jupyter_extension/webio-jupyter-labextension/labextension.ts

@@ -8,6 +8,7 @@ import type { DocumentRegistry } from "@jupyterlab/docregistry";
 import type { INotebookModel, NotebookPanel } from "@jupyterlab/notebook";
 import type { IRenderMime } from "@jupyterlab/rendermime";
 import type { Kernel } from "@jupyterlab/services";
+import type { IKernelConnection } from "@jupyterlab/services/lib/kernel/kernel";
 
 import WebIO from "@webio/webio";
 
@@ -222,6 +223,19 @@ class WebIONotebookManager {
       log("Received WebIO comm message:", msg);
       this._webIO.dispatch(msg.content.data);
     };
+    kernel.statusChanged.connect(
+      (kernel: IKernelConnection, stat: Kernel.Status) => {
+        if (
+          stat === "restarting" ||
+          stat === "autorestarting" ||
+          stat === "dead"
+        ) {
+          // Stop attempting to handle callbacks if previous kernel is gone
+          this._webIO.setSendCallback((msg: any) => {});
+        }
+      },
+      this,
+    );
 
     this.setWebIOMetadata(kernel.id, this.comm.commId);
   }