ClassCastException because of different classloaders

Hello,

In some place of my plugin I try to get class declared in a file by this way:

class MyCustomTestContributor: CompletionContributor() {
    override fun fillCompletionVariants(parameters: CompletionParameters, result: CompletionResultSet) {
        val aClass = parameters.originalFile.children[4] as KtClass //fails here
    }
}

This code fails with

java.lang.ClassCastException: org.jetbrains.kotlin.psi.KtClass cannot be cast to org.jetbrains.kotlin.psi.KtClass

because of KtClass was loaded by different classloaders:
1. PluginClassLoader[org.jetbrains.kotlin, 1.3.30-release-IJ2018.3-1] com.intellij.ide.plugins.cl.PluginClassLoader@46ccae7e
2. PluginClassLoader[my.project.id, 1.0-SNAPSHOT] com.intellij.ide.plugins.cl.PluginClassLoader@74061aa0

I have <depends>org.jetbrains.kotlin</depends> in my plugin.xml.
I have com.github.cretz.kastree:kastree-ast-psi:0.4.0 in my gradle dependencies. Build script attached bellow.
Kastree uses kotlin-compiler-embedabble.jar, I thought it can clash with Kotlin plugin libraries, so I tried to exclude kotlin-compiler-embedabble from my classpath (as transitive depencency) and include it as compileOnly depenency, but in this case IDEA throws the exception on startup: 

java.lang.NoClassDefFoundError: org/jetbrains/kotlin/cli/jvm/compiler/KotlinCoreEnvironment

I also tried to add kotlin-compiler-embedabble.jar into Intellij Platform SDK classpath but it doesn't work.

My gradle build script looks like: 

plugins {
    id 'org.jetbrains.intellij' version '0.4.7'
    id 'org.jetbrains.kotlin.jvm' version '1.3.30'
}

group 'my.plugin.group'
version '1.0-SNAPSHOT'

repositories {
    mavenCentral()
    jcenter()
    flatDir {
        dirs 'libs'
    }
}

dependencies {
    implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk8"
    compile('com.github.cretz.kastree:kastree-ast-psi:0.4.0')
    compile 'com.fasterxml.jackson.core:jackson-core:2.9.8'
    compile "org.jetbrains.kotlinx:kotlinx-html-jvm:0.6.12"
    compile (name:'jfxrt', ext:'jar')
}

intellij {
    version '2018.3.4'
    plugins 'org.jetbrains.kotlin:1.3.30-release-IJ2018.3-1'
    intellij.updateSinceUntilBuild false
}

patchPluginXml {
    changeNotes """
      Add change notes here.<br>
      <em>most HTML tags may be used</em>"""
}

What would you suggest to resolve this issue? Thank you!