Remove all text when cutting in the composer

src/components/views/rooms/BasicMessageComposer.js

@@ -209,8 +209,8 @@ export default class BasicMessageEditor extends React.Component {
             const selectedParts = range.parts.map(p => p.serialize());
             event.clipboardData.setData("application/x-riot-composer", JSON.stringify(selectedParts));
             if (type === "cut") {
-                selection.deleteFromDocument();
-                range.replace([]);
+                // Remove the text, updating the model as appropriate
+                replaceRangeAndMoveCaret(range, []);
             }
             event.preventDefault();
         }

src/editor/position.js

@@ -117,7 +117,7 @@ export default class DocumentPosition {
         }
         offset += this.offset;
         const lastPart = model.parts[this.index];
-        const atEnd = offset >= lastPart.text.length;
+        const atEnd = !lastPart || offset >= lastPart.text.length; // if no last part, we're at the end
         return new DocumentOffset(offset, atEnd);
     }