[BUG][tyescript-axios] withSeparateModelsAndApi option throws exception

Created by: boenrobot

Bug Report Checklist

Have you provided a full/minimal spec to reproduce the issue?
Have you validated the input using an OpenAPI validator (example)?
What's the version of OpenAPI Generator used?
Have you search for related issues/PRs?
What's the actual output vs expected output?

Description

I would like to generate a typescript-axios client with separate models. I can successfully generate one without separate ones, but with separate ones, an exception is thrown.

openapi-generator version

4.2.2

OpenAPI declaration file content or url

The spec is dynamically generated by a Nest application (the very reason I want to use a generator to begin with is to keep it in sync). Here's the one that produces this error:

{
  "openapi": "3.0.0",
  "info": {
    "title": "Frontoffice",
    "description": "API docs",
    "version": "0.1",
    "contact": {}
  },
  "tags": [],
  "servers": [],
  "components": {
    "schemas": {
      "RegisterRequestDto": {
        "type": "object",
        "properties": {
          "username": {
            "type": "string",
            "pattern": "^[a-zA-Z0-9]+([a-zA-Z0-9][_\\-][a-zA-Z0-9])*[a-zA-Z0-9]+$"
          },
          "password": {
            "type": "string",
            "minLength": 8
          },
          "email": {
            "type": "string",
            "format": "email"
          },
          "phone": {
            "type": "string",
            "description": "Phone number. Can contain arbitrary symbols for readability, but only the digits, and an optional leading plus are accepted.",
            "pattern": "[0-9]+"
          }
        },
        "required": [
          "username",
          "password",
          "email",
          "phone"
        ]
      },
      "PlayerInsertResultDto": {
        "type": "object",
        "properties": {
          "playerId": {
            "type": "number"
          }
        },
        "required": [
          "playerId"
        ]
      },
      "ConflictException": {
        "type": "object",
        "properties": {}
      },
      "UnauthorizedException": {
        "type": "object",
        "properties": {}
      }
    }
  },
  "paths": {
    "/": {
      "get": {
        "operationId": "ApiController-getIndex",
        "responses": {
          "200": {
            "description": ""
          }
        }
      }
    },
    "/register": {
      "get": {
        "operationId": "RegisterApiController-getIndex",
        "responses": {
          "200": {
            "description": ""
          }
        }
      },
      "post": {
        "operationId": "RegisterApiController-postIndex",
        "parameters": [],
        "requestBody": {
          "required": true,
          "content": {
            "application/json": {
              "schema": {
                "$ref": "#/components/schemas/RegisterRequestDto"
              }
            }
          }
        },
        "responses": {
          "201": {
            "description": "",
            "content": {
              "application/json": {
                "schema": {
                  "$ref": "#/components/schemas/PlayerInsertResultDto"
                }
              }
            }
          },
          "409": {
            "description": "If the player already exists",
            "content": {
              "application/json": {
                "schema": {
                  "$ref": "#/components/schemas/ConflictException"
                }
              }
            }
          }
        }
      }
    },
    "/login": {
      "get": {
        "operationId": "LoginApiController-getIndex",
        "responses": {
          "200": {
            "description": ""
          }
        }
      },
      "post": {
        "operationId": "LoginApiController-postIndex",
        "responses": {
          "201": {
            "description": "A sanitized player object"
          },
          "401": {
            "description": "On invalid credentials",
            "content": {
              "application/json": {
                "schema": {
                  "$ref": "#/components/schemas/UnauthorizedException"
                }
              }
            }
          }
        }
      }
    },
    "/_schema": {
      "get": {
        "operationId": "SwaggerApiController-getSchema",
        "responses": {
          "200": {
            "description": ""
          }
        }
      }
    },
    "/openapi.json": {
      "get": {
        "operationId": "SwaggerApiController-getDocument",
        "responses": {
          "200": {
            "description": ""
          }
        }
      }
    }
  }
}

Command line used for generation

npx openapi-generator generate -i openapi.json --config .\openapi-generate.json -g typescript-axios -o openapi

Steps to reproduce

Set withSeparateModelsAndApi to true in the --config file.
Run the command above command.

Output in console:

Exception in thread "main" java.lang.RuntimeException: apiPackage and modelPackage must be defined
        at org.openapitools.codegen.languages.TypeScriptAxiosClientCodegen.processOpts(TypeScriptAxiosClientCodegen.java:117)
        at org.openapitools.codegen.DefaultGenerator.configureGeneratorProperties(DefaultGenerator.java:196)
        at org.openapitools.codegen.DefaultGenerator.generate(DefaultGenerator.java:913)
        at org.openapitools.codegen.cmd.Generate.run(Generate.java:416)
        at org.openapitools.codegen.OpenAPIGenerator.main(OpenAPIGenerator.java:61)

as opposed to withSeparateModelsAndApi set to false, which finishes with no errors.