🪲 Remove a source of client exceptions (#5812)

A lot of client exceptions were originating from the embedded editor, trying to call `removeAttribute()` on an object that happened to be `null`.

The reason was an assertion that the result of `getElementById()` would never be `null`, where in fact, it was.

Replace with a null coalescing function call `?.removeAttribute()` so that the function call falls to the floor if the element doesn't exist.

**How to test**

Visit http://localhost:8080/embedded/1/?lang=nl&run=true&readOnly=false&fullWidth=false&program=IyBTY2hyaWpmIGpvdXcgY29kZSBoaWVy%22 and run any program. Observe that no exceptions get logged to the JavaScript console.

(cc @jpelay, I spied you [here](#5673) 😛 this is why `!` is dangerous, even you *think* this can never happen there's always a case you didn't think of -- better to program defensively or fail real hard with an explicit error message)

static/js/appbundle.js

@@ -104201,14 +104201,15 @@ def note_with_error(value, err):
     }
   }
   function stopDebug() {
+    var _a3;
     if (step_debugger === true) {
       var debugButton = $("#debug_button");
       debugButton.show();
       var continueButton = $("#debug_continue");
       var stopButton = $("#debug_stop");
       var resetButton = $("#debug_restart");
       var runButtonContainer = $("#run_button_container");
-      document.getElementById("debug_continue").removeAttribute("disabled");
+      (_a3 = document.getElementById("debug_continue")) == null ? void 0 : _a3.removeAttribute("disabled");
       $("#stopit").hide();
       $("#runit").show();
       runButtonContainer.show();
@@ -104231,7 +104232,8 @@ def note_with_error(value, err):
     }
   }
   function incrementDebugLine() {
-    document.getElementById("debug_continue").removeAttribute("disabled");
+    var _a3;
+    (_a3 = document.getElementById("debug_continue")) == null ? void 0 : _a3.removeAttribute("disabled");
     const active_suspension = theGlobalDebugger.getActiveSuspension();
     const suspension_info = theGlobalDebugger.getSuspensionInfo(active_suspension);
     const lineNumber = suspension_info.lineno;

static/js/debugging.ts

@@ -60,7 +60,7 @@ export function toggleVariableView(){
   if($('#variables #variable_list li').length == 0){
     $('#variable_button').hide();
   }
-  else{    
+  else{
     $('#variable_button').show();
     $('#variable_list').show();
     document.getElementById('variables_arrow')!.classList.remove('fa-angle-up');
@@ -171,7 +171,7 @@ export function initializeDebugger(options: InitializeDebuggerOptions) {
 
   if(options.level != 0){
     let level = options.level;
-    variable_view = level >= 2;  
+    variable_view = level >= 2;
   }
   initializeBreakpoints(options.editor);
 }
@@ -270,7 +270,7 @@ export function stopDebug() {
     var stopButton = $('#debug_stop');
     var resetButton = $('#debug_restart');
     var runButtonContainer = $('#run_button_container');
-    document.getElementById('debug_continue')!.removeAttribute('disabled');
+    document.getElementById('debug_continue')?.removeAttribute('disabled');
     $('#stopit').hide();
     $('#runit').show()
     runButtonContainer.show();
@@ -298,7 +298,7 @@ function clearDebugVariables() {
 }
 
 export function incrementDebugLine() {
-  document.getElementById('debug_continue')!.removeAttribute('disabled');
+  document.getElementById('debug_continue')?.removeAttribute('disabled');
   const active_suspension = theGlobalDebugger.getActiveSuspension();
   const suspension_info = theGlobalDebugger.getSuspensionInfo(active_suspension);
   const lineNumber = suspension_info.lineno;
@@ -321,17 +321,17 @@ export function incrementDebugLine() {
     const startingLine = map.python_range.from_line + theGlobalDebugger.get_code_starting_line();
     const finishingLine = map.python_range.to_line + theGlobalDebugger.get_code_starting_line();
     // Maybe we hit the correct mapping for this line
-    if (lineNumber >= startingLine && lineNumber <= finishingLine) { 
+    if (lineNumber >= startingLine && lineNumber <= finishingLine) {
       // Highlight whole line if it's a full command
-      if(fullLineCommands.includes(map.command)){        
-        // lines in ace start at 0       
+      if(fullLineCommands.includes(map.command)){
+        // lines in ace start at 0
         const lines = theGlobalEditor.contents.split('\n');
         const line = lines[map.hedy_range.from_line - 1];
         const ifMatches = ifRe.exec(line);
         const repeatMatches = repeatRe.exec(line);
         const elseMatches = elseRe.exec(line);
         if (ifMatches || repeatMatches || elseMatches) {
-          theGlobalEditor.setDebuggerCurrentLine(map.hedy_range.from_line, 
+          theGlobalEditor.setDebuggerCurrentLine(map.hedy_range.from_line,
             map.hedy_range.from_column, map.hedy_range.to_column - 1);
         } else {
           theGlobalEditor.setDebuggerCurrentLine(map.hedy_range.from_line);
@@ -345,20 +345,20 @@ export function incrementDebugLine() {
         } else {
           const fullLine = lines[map.hedy_range.from_line - 1];
           line = fullLine.substring(map.hedy_range.from_column - 1, map.hedy_range.to_column - 1);
-        }       
+        }
         const activeLine: string = theGlobalDebugger.get_source_line(lineNumber - 1);
 
         if (activeLine.match(/ *if/)) {
           const ifMatches = ifRe.exec(line);
           if (ifMatches) {
             const length = ifMatches[1].length;
-            theGlobalEditor.setDebuggerCurrentLine(map.hedy_range.from_line, map.hedy_range.from_column, map.hedy_range.from_column + length - 1);            
+            theGlobalEditor.setDebuggerCurrentLine(map.hedy_range.from_line, map.hedy_range.from_column, map.hedy_range.from_column + length - 1);
             break
           }
         } else if (activeLine.match(/ *for/)) {
           const repeatMatches = repeatRe.exec(line);
-          if (repeatMatches){            
-            const length = repeatMatches[1].length;            
+          if (repeatMatches){
+            const length = repeatMatches[1].length;
             theGlobalEditor.setDebuggerCurrentLine(map.hedy_range.from_line, map.hedy_range.from_column, map.hedy_range.from_column + length - 1);
             break
           }