Wait for the session to be ready when executing a command without opening the tab (#279)

* Await for the session to be ready

* Apply prettier

* Fix code diff in  README.md

* Fix code reference in README.md

Author: cmarmo

kernel-output/README.md

@@ -195,7 +195,7 @@ You can then add the commands to the palette by iterating
 on a list:
 
 ```ts
-// src/index.ts#L100-L103
+// src/index.ts#L104-L107
 
 // add items in command palette and menu
 [CommandIDs.create, CommandIDs.execute].forEach(command => {
@@ -240,16 +240,12 @@ to be executed by the kernel. Then you will send it to your panel for execution
 and display:
 
 ```ts
-// src/index.ts#L78-L98
+// src/index.ts#L78-L102
 
 commands.addCommand(CommandIDs.execute, {
   label: trans.__('Contact Kernel and Execute Code'),
   caption: trans.__('Contact Kernel and Execute Code'),
   execute: async () => {
-    // Create the panel if it does not exist
-    if (!panel) {
-      await createPanel();
-    }
     // Prompt the user about the statement to be executed
     const input = await InputDialog.getText({
       title: trans.__('Code to execute'),
@@ -259,7 +255,15 @@ commands.addCommand(CommandIDs.execute, {
     // Execute the statement
     if (input.button.accept) {
       const code = input.value || '';
-      panel.execute(code);
+      if (!panel) {
+        // Create the panel if it does not exist
+        createPanel().then(async panel => {
+          await panel.session.ready;
+          panel.execute(code);
+        });
+      } else {
+        panel.execute(code);
+      }
     }
   }
 });

kernel-output/src/index.ts

@@ -79,10 +79,6 @@ function activate(
     label: trans.__('Contact Kernel and Execute Code'),
     caption: trans.__('Contact Kernel and Execute Code'),
     execute: async () => {
-      // Create the panel if it does not exist
-      if (!panel) {
-        await createPanel();
-      }
       // Prompt the user about the statement to be executed
       const input = await InputDialog.getText({
         title: trans.__('Code to execute'),
@@ -92,7 +88,15 @@ function activate(
       // Execute the statement
       if (input.button.accept) {
         const code = input.value || '';
-        panel.execute(code);
+        if (!panel) {
+          // Create the panel if it does not exist
+          createPanel().then(async panel => {
+            await panel.session.ready;
+            panel.execute(code);
+          });
+        } else {
+          panel.execute(code);
+        }
       }
     }
   });