message: disable "Selected" button when empty
diff: @@ -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>
  