Fix rare cleanup problem in TGeo

Author: linev

modules/geom/TGeoPainter.mjs

@@ -4419,14 +4419,14 @@ class TGeoPainter extends ObjectPainter {
             if (obj.fVolume && obj.fVolume.$geo_painter===this) delete obj.fVolume.$geo_painter;
          }
 
-         if (this._main_painter) {
+         if (this._main_painter?._slave_painters) {
             let pos = this._main_painter._slave_painters.indexOf(this);
             if (pos >= 0) this._main_painter._slave_painters.splice(pos,1);
          }
 
-         for (let k = 0; k < this._slave_painters.length;++k) {
+         for (let k = 0; k < this._slave_painters?.length; ++k) {
             let slave = this._slave_painters[k];
-            if (slave && (slave._main_painter===this)) slave._main_painter = null;
+            if (slave && (slave._main_painter === this)) slave._main_painter = null;
          }
 
          delete this.geo_manager;