SwingPanel throws NPE on focus request if nothing focusable

[jasonsparc]

Kotlin: 1.7.20
Compose: 1.2.1

If there's no component focusable inside a SwingPanel and you try to request focus to it via, e.g., tabbing, it'll crash the application.

Try the following sample code,

import androidx.compose.foundation.layout.*
import androidx.compose.material.OutlinedTextField
import androidx.compose.material.Text
import androidx.compose.runtime.Composable
import androidx.compose.runtime.mutableStateOf
import androidx.compose.runtime.remember
import androidx.compose.ui.Modifier
import androidx.compose.ui.awt.SwingPanel
import androidx.compose.ui.unit.dp
import androidx.compose.ui.window.singleWindowApplication
import javax.swing.JPanel

fun main() = singleWindowApplication {
    val letsSetItToFocusable = false
    Column(
        modifier = Modifier.padding(50.dp).fillMaxSize(),
        verticalArrangement = Arrangement.spacedBy(8.dp),
    ) {
        SomethingToFocusOn()
        SwingPanel(
            factory = {
                JPanel().apply {
                    if (letsSetItToFocusable) isFocusable = true
                }
            },
            modifier = Modifier.fillMaxWidth().height(40.dp),
        )
        SomethingToFocusOn()
    }
}

@Composable
fun SomethingToFocusOn() {
    val text = remember { mutableStateOf("") }
    OutlinedTextField(value = text.value, onValueChange = { text.value = it }, singleLine = true, label = {
        Text("Try tab to switch focus")
    })
}

Outcome:

Relevant stack trace:

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException: Cannot invoke "java.awt.Component.requestFocus(java.awt.event.FocusEvent$Cause)" because the return value of "java.awt.FocusTraversalPolicy.getFirstComponent(java.awt.Container)" is null
	at androidx.compose.ui.awt.FocusSwitcher$Content$1.invoke(SwingPanel.desktop.kt:191)
	at androidx.compose.ui.awt.FocusSwitcher$Content$1.invoke(SwingPanel.desktop.kt:186)
	at androidx.compose.ui.focus.FocusChangedModifierKt$onFocusChanged$2$1.invoke(FocusChangedModifier.kt:47)
	at androidx.compose.ui.focus.FocusChangedModifierKt$onFocusChanged$2$1.invoke(FocusChangedModifier.kt:44)
	at androidx.compose.ui.focus.FocusEventModifierLocal.propagateFocusEvent(FocusEventModifier.kt:137)
	at androidx.compose.ui.focus.FocusTransactionsKt.sendOnFocusEvent(FocusTransactions.kt:279)
	at androidx.compose.ui.focus.FocusModifier.setFocusState(FocusModifier.kt:79)
	...

---

Now, in the above sample code, set letsSetItToFocusable = true instead, and it won't crash anymore. That's because the SwingPanel now has a focusable component.

Digging into the source code of SwingPanel, I found the following lines to be problematic:

info.container.focusTraversalPolicy
    .getFirstComponent(info.container)
    .requestFocus(FocusEvent.Cause.TRAVERSAL_FORWARD)

https://github.com/JetBrains/androidx/blob/release/1.2.1/compose/ui/ui/src/desktopMain/kotlin/androidx/compose/ui/awt/SwingPanel.desktop.kt#L190

info.container.focusTraversalPolicy
    .getLastComponent(info.container)
    .requestFocus(FocusEvent.Cause.TRAVERSAL_FORWARD)

https://github.com/JetBrains/androidx/blob/release/1.2.1/compose/ui/ui/src/desktopMain/kotlin/androidx/compose/ui/awt/SwingPanel.desktop.kt#L203

The problem is, both getFirstComponent(…) and getLastComponent(…) of the FocusTraversalPolicy interface are allowed to return null, and yet the kotlin code is treating it that it won't. This was confirmed via a debugger, and also if you use the IDE to look for implementations of the FocusTraversalPolicy interface, you will find that it may indeed return null when no swing component is focusable.

Suggestion: Honor the nullability of those methods, and perhaps simply skip over the SwingPanel if it has no focusable components when looking for components to focus to.