d2ir: fix panic with class overlay

d2compiler/compile_test.go

@@ -1714,6 +1714,40 @@ steps: {
 				assert.Equal(t, 1, len(g.Layers[0].Steps))
 			},
 		},
+		{
+			name: "import-classes-boards",
+
+			text: `classes: {
+  a: {
+		label: hi
+  }
+}
+
+layers: {
+  asdf: {
+    qwer: {
+      layers: {
+        ok: {
+          bok
+				}
+      }
+    }
+  }
+  wert: {
+    classes: @classes
+  }
+}
+
+`,
+			files: map[string]string{
+				"classes.d2": `
+c: {
+	label: bye
+}
+`,
+			},
+			expErr: `d2/testdata/d2compiler/TestCompile/import-classes-boards.d2:10:7: layers is only allowed at a board root`,
+		},
 		{
 			name: "import_url_link",
 

d2ir/compile.go

@@ -113,7 +113,7 @@ func (c *compiler) overlayClasses(m *Map) {
 		if lClasses == nil {
 			lClasses = classes.Copy(l).(*Field)
 			l.Fields = append(l.Fields, lClasses)
-		} else {
+		} else if lClasses.Map() != nil {
 			base := classes.Copy(l).(*Field)
 			OverlayMap(base.Map(), lClasses.Map())
 			l.DeleteField("classes")

testdata/d2compiler/TestCompile/import-classes-boards.exp.json

@@ -0,0 +1,11 @@
+{
+  "graph": null,
+  "err": {
+    "errs": [
+      {
+        "range": "d2/testdata/d2compiler/TestCompile/import-classes-boards.d2,9:6:75-9:12:81",
+        "errmsg": "d2/testdata/d2compiler/TestCompile/import-classes-boards.d2:10:7: layers is only allowed at a board root"
+      }
+    ]
+  }
+}