handle null dereference in imagemethods + nicer panic

sim/libmbit.ts

@@ -78,8 +78,44 @@ namespace pxsim {
         }
     }
     
+    /**
+      * Error codes used in the micro:bit runtime.
+      */
+    export enum PanicCode {
+        // PANIC Codes. These are not return codes, but are terminal conditions.
+        // These induce a panic operation, where all code stops executing, and a panic state is
+        // entered where the panic code is diplayed.
+
+        // Out out memory error. Heap storage was requested, but is not available.
+        MICROBIT_OOM = 20,
+
+        // Corruption detected in the micro:bit heap space
+        MICROBIT_HEAP_ERROR = 30,
+
+        // Dereference of a NULL pointer through the ManagedType class,
+        MICROBIT_NULL_DEREFERENCE = 40,
+    };
+
     export function panic(code: number) {
         console.log("PANIC:", code)
+        led.setBrightness(255);
+        let img = board().image;
+        img.clear();
+        img.set(0, 4, 255);
+        img.set(1, 3, 255);
+        img.set(2, 3, 255);
+        img.set(3, 3, 255);
+        img.set(4, 4, 255);
+        img.set(0, 0, 255);
+        img.set(1, 0, 255);
+        img.set(0, 1, 255);
+        img.set(1, 1, 255);
+        img.set(3, 0, 255);
+        img.set(4, 0, 255);
+        img.set(3, 1, 255);
+        img.set(4, 1, 255);
+        runtime.updateDisplay();
+       
         throw new Error("PANIC " + code)
     }
 
@@ -515,30 +551,41 @@ namespace pxsim.images {
 }
 
 namespace pxsim.ImageMethods {
-    export function showImage(i: Image, offset: number) {
-        // TODO offset?
-        i.copyTo(0, 5, board().image, 0)
-        runtime.queueDisplayUpdate()
-    }
+    export function showImage(leds: Image, offset: number) {
+        if (!leds) panic(PanicCode.MICROBIT_NULL_DEREFERENCE);        
         
-    export function plotImage(leds: Image, offset:number): void {
         leds.copyTo(offset, 5, board().image, 0)
         runtime.queueDisplayUpdate()
     }
 
-    export function clear(i: Image) {
-        i.clear();
+    export function plotImage(leds: Image, offset: number): void {
+        if (!leds) panic(PanicCode.MICROBIT_NULL_DEREFERENCE);
+        
+        leds.copyTo(offset, 5, board().image, 0)
+        runtime.queueDisplayUpdate()
+    }
+
+    export function clear(leds: Image) {
+        if (!leds) panic(PanicCode.MICROBIT_NULL_DEREFERENCE);        
+        
+        leds.clear();
     }
 
     export function setPixelBrightness(i: Image, x: number, y: number, b: number) {
+        if (!i) panic(PanicCode.MICROBIT_NULL_DEREFERENCE);        
+
         i.set(x, y, b);
     }
 
     export function pixelBrightness(i: Image, x: number, y: number): number {
+        if (!i) panic(PanicCode.MICROBIT_NULL_DEREFERENCE);        
+
         return i.get(x, y);
     }
 
     export function scrollImage(leds: Image, interval: number, stride: number): void {
+        if (!leds) panic(PanicCode.MICROBIT_NULL_DEREFERENCE);        
+        
         let cb = getResume()
         let off = stride > 0 ? 0 : leds.width - 1;
         let display = board().image;