Illegal reference to private object

I created a Blueprint descendant based on Canvas Render Target 2D (BP_Canvas). I added custom logic to it and after that I created an asset (RT_Heatmap) using Editor Utility Blueprint, the code is on the screenshot, after that I can’t save RT_Heatmap. When I save, a window appears with the message:
Failed to save asset “/Game/UI/Texture/RT_Heatmap” (RT_Heatmap.uasset).

Cancel: Stop saving all assets and return to the editor.
Retry: Try saving the asset again.
Continue: Skip saving this asset only.
If I click Continue I get the error:
The following assets failed to save correctly:
/Game/UI/Texture/RT_Heatmap
And in the log:
LogSavePackage: Warning: references to AssetImportData/Game/UI/BP_Canvas.Default__BP_Canvas_C:AssetImportData:
LogSavePackage: warning: BP_Canvas_C /Game/UI/BP_Canvas.Default__BP_Canvas_C (1 references)
LogSavePackage: Warning: 0) ObjectProperty/Script/Engine.Texture:AssetImportData
Warning: Unable to save ‘/Content/UI/Texture/RT_Heatmap.uasset’: Invalid reference to private object: ‘AssetImportData’. /Game/UI/BP_Canvas.Default__BP_Canvas_C:AssetImportData’, which is referenced by ‘Default__BP_Canvas_C’ (in ‘/Game/UI/BP_Canvas’) in its AssetImportData property.
How can this be fixed or what did I do wrong?

This usually happens when you have more than one editor open.