check that document exists before mounting debug panel

src/client/client.js

@@ -279,16 +279,19 @@ class _ClientImpl {
       this.debug !== false &&
       this._debugPanel == null
     ) {
-      let target = document.body;
-      if (this.debug && this.debug.target) {
+      let target = document && document.body;
+      if (this.debug && this.debug.target !== undefined) {
         target = this.debug.target;
       }
-      this._debugPanel = new debugImpl({
-        target,
-        props: {
-          client: this,
-        },
-      });
+
+      if (target) {
+        this._debugPanel = new debugImpl({
+          target,
+          props: {
+            client: this,
+          },
+        });
+      }
     }
   }
 

src/client/client.test.js

@@ -642,6 +642,13 @@ describe('start / stop', () => {
     client.stop();
   });
 
+  test('no error when mounting on null element', () => {
+    const client = Client({ game: {}, debug: { target: null } });
+    client.start();
+    client.stop();
+    expect(client._debugPanel).toBe(null);
+  });
+
   test('override debug implementation', () => {
     const client = Client({ game: {}, debug: { impl: Debug } });
     client.start();