异常：

Schema export directory is not provided to the annotation processor so we cannot export the schema. You can either provide `room.schemaLocation` annotation processor argument OR set exportSchema to false.

解决：模块的build.gradle文件中加入

android {
    ...
    defaultConfig {
        ...
        // Groovy版本
        javaCompileOptions {
            annotationProcessorOptions {
                arguments += [
                    "room.schemaLocation":"$projectDir/schemas".toString(),
                    "room.incremental":"true"
                    ]
            }
        }

        // kotlin版本
        javaCompileOptions {
            annotationProcessorOptions {
                arguments += mapOf(
                    "room.schemaLocation" to "$projectDir/schemas",
                    "room.incremental" to "true"
                )
            }
        }
    }
}