Clean your Visual Studio(Open the Cmd window > to the “C:\\Program Files (x86)\\Microsoft Visual Studio\\2019\\Preview\\Common7\\IDE” where installed VS > Input the command(devenv /resetuserdata) > restart the Visual Studio)
you can also try
You can add the missing controls by below steps.
a. Right click the blank of any Tab, then choose the “Choose items…”.
b.Choose one tab on the “Choose Toolbox items” dialog.
c. Chose the controls that you want to add.
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