The cause: I think this error occurs if you frequently edit keys, this warning will appear owing to security features; do not disable this feature. This warning only appeared once. If the problem persists after a second connection, the issue is most likely with writing to the known_hosts file. After that, you’ll receive the following message:

Failed to add the host to the list of known hosts 

The solution: To solve this problem you can change the file’s owner or make it writable by your user by fixing the file’s permissions.

sudo chown -v $USER ~/.ssh/known_hosts

The cause: The cause of this error perhaps is due to using [formControl] instead of [formGroup]

The solution: To solve this error, you should change [formControl] to [formGroup]

This is a wrong code:

@Component({
  selector: 'app-application-purpose',
  template: `
  <div [formControl]="formGroup"> <!-- '[formControl]' IS THE WRONG ATTRIBUTE -->
    <input formControlName="formGroupProperty" />
  </div>
  `
})
export class MyComponent implements OnInit {
  formGroup: FormGroup

  constructor(
    private formBuilder: FormBuilder 
  ) { }

  ngOnInit() {
    this.formGroup = this.formBuilder.group({
      formGroupProperty: ''
   })
  }
}

I have changed this to the right code as below:

@Component({
  selector: 'app-application-purpose',
  template: `
    <div [formGroup]="formGroup"> <!-- '[formGroup]' IS THE RIGHT ATTRIBUTE -->
      <input formControlName="formGroupProperty" />
    </div>
  `
 })
 export class MyComponent implements OnInit {
   formGroup: FormGroup

   constructor(
     private formBuilder: FormBuilder
   ) { }
   ngOnInit() {
     this.formGroup = this.formBuilder.group({
       formGroupProperty: '' 
     })
   }
 }

The cause: This error happens because of within the onClick as the setAllResultsVisible is called, it will trigger state change and result on every render

The solution:  Before setAllResultsVisible, you should change your onlick event add()=> 

<p onClick={() => setAllResultsVisible(!allResultsVisible) }>
    More results v
</p>

The cause: You have to  create the database as a container database instead of creating user in the container, i.e. CDB$ROOT, however, i think you should create the user in the PLUGGABLE database.

The solution: You can resolve the error by  creating a “pluggable user” aka “local user”.
Local users belong only to a single PDB. These users may be given administrative privileges, but only for that PDB inside which they exist. For that, you should connect to pluggable datable like that:

alter session set container = nameofyourpluggabledatabase;

and now, you can create user like usually:

create user username identified by password;