disable "Selected" button when empty

src/mol-plugin-ui/structure/selection.tsx

@@ -152,11 +152,14 @@ export class StructureSelectionControls<P, S extends StructureSelectionControlsS
     }
 
     renderControls() {
+        const stats = this.plugin.managers.structure.selection.stats
+        const empty = stats.structureCount === 0 || stats.elementCount === 0;
+
         return <>
             <ParameterControls params={StructureSelectionParams} values={this.values} onChangeObject={this.setProps} />
             {this.controls}
             <div className='msp-control-row msp-row-text' style={{ marginTop: '6px' }}>
-                <button className='msp-btn msp-btn-block' onClick={this.focus} title='Click to Focus Selection'>
+                <button className='msp-btn msp-btn-block' onClick={this.focus} title='Click to Focus Selection' disabled={empty}>
                     <Icon name='focus-on-visual' style={{ position: 'absolute', left: '5px' }} />
                     {this.stats}
                 </button>