Hi Vinitha,
After installing @syncfusion/ej2-richtexteditor component the error disappeared.
Let me explain:
1. I tried to create an example on StackBlitz, using only "@syncfusion/ej2-angular-richtexteditor";
2. I adjusted the toolbar CSS with:
@import '~@syncfusion/ej2-base/styles/material.css';
@import '~@syncfusion/ej2-icons/styles/material.css';
@import '~@syncfusion/ej2-buttons/styles/material.css';
@import '~@syncfusion/ej2-splitbuttons/styles/material.css';
@import '~@syncfusion/ej2-inputs/styles/material.css';
@import '~@syncfusion/ej2-lists/styles/material.css';
@import '~@syncfusion/ej2-navigations/styles/material.css';
@import '~@syncfusion/ej2-popups/styles/material.css';
@import '~@syncfusion/ej2-richtexteditor/styles/material.css';
3. StackBlitz required the ej2 modules to import. So, I installed the @syncfusion/ej2-richtexteditor and the orthers.
4. And puff... the error disappeared in the StackBlitz project.
5. I tried installing @syncfusion/ej2-richtexteditor on my original project and fixed the problem.
So, my conclusion is: installing only @syncfusion/ej2-angular-richtexteditor to use the rich text editor works, but need correction.