Redo, Undo disabled when no graphs loaded

Author: Rahuljagwani

src/toolbarActions/toolbarList.js

@@ -75,7 +75,7 @@ const toolbarList = (state, dispatcher) => [
         text: 'Undo',
         icon: FaUndo,
         action: undo,
-        active: state.undoEnabled,
+        active: state.undoEnabled && state.curGraphInstance,
         visibility: true,
         hotkey: 'Ctrl+Z',
     },
@@ -84,7 +84,7 @@ const toolbarList = (state, dispatcher) => [
         text: 'Redo',
         icon: FaRedo,
         action: redo,
-        active: state.redoEnabled,
+        active: state.redoEnabled && state.curGraphInstance,
         visibility: true,
         hotkey: 'Ctrl+Shift+Z,Ctrl+Y',
     },