Install libqscintilla2-designer by entering the following commands in the terminal:
sudo apt update sudo apt install libqscintilla2-designer
Qt4 Designer plugin for QScintilla 2 (Transitional package)
QScintilla is a text editor for Qt4 with features especially useful when writing and debugging source code. These include support for syntax styling, error indicators, code completion, call tips and margins. . This package contains a plugin to use the QScintilla 2 widget in the Qt4 UI Designer. . This is a dummy transitional package provided to support upgrades.