Can Anyone tell me why this is coming?

[Rohit_Singh2 (Bharataai)]

I had just created a simple app . And whenever i open my application this error shows??

[golumaths100 (Shivam Chouhan)]

Which extension are you using?
Also show your blocks.

[ramrajput200021 (Techy Rajput)]

Please tell more information
And read this -

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[The_K_Studio (Karan M.)]

Are you using colin tree list view extension?