added error checking and updated demo

ts/docx/run/run-components/drawing/index.ts

@@ -7,6 +7,10 @@ export class Drawing extends XmlComponent {
     constructor(imageData: IData) {
         super("w:drawing");
 
+        if (imageData === undefined) {
+            throw new Error("imageData cannot be undefined");
+        }
+
         this.root.push(new Inline(imageData.referenceId));
     }
 }

ts/media/index.ts

@@ -9,8 +9,14 @@ export class Media {
         this.map = new Map<string, IData>();
     }
 
-    public getMedia(key: string): IData | undefined {
-        return this.map.get(key);
+    public getMedia(key: string): IData {
+        const data = this.map.get(key);
+
+        if (data === undefined) {
+            throw new Error(`Cannot find image with the key ${key}`);
+        }
+
+        return data;
     }
 
     public addMedia(key: string, filePath: string): void {